Can A Laser Damage A Camera

1.

Introduction

Since the invention of the laser in 1960, laser systems have go more and more powerful and even more than meaty from year to year. Laser radiation is increasingly becoming a hazard to the human eye as well as to electro-optical imaging sensors—not only in form of laser impairment only also due to light amplification by stimulated emission of radiation dazzle. Knowing that detectors, such equally complementary metal–oxide–semiconductors (CMOS) or charge-coupled devices (CCD), are very sensitive to laser light, there is a stiff continued interest in protection measures against dazzling and damaging. At our institute, a lot of effort was put into the investigation of laser protection measures that provide wavelength-independent protection.^{1 , 2} 1 of our concepts to suppress laser dazzle in camera systems is based on installing a digital micromirror device (DMD) into a focal plane of an optical setup in combination with wavelength multiplexing.^{3 – 5} A scheme of this optical setup and a photograph of the hardened sensor according to this concept are shown in Fig. 1. Without going into the details of this optical setup, ane tin see that 2 devices are placed at focal planes: (a) the DMD at the intermediate focal plane and (b) the imaging sensor in the focal airplane of the camera lens. Now the question arises as to what would happen if this sensor is exposed to laser irradiation with intensities far beyond the threshold for laser dazzle. Which device will be damaged first? The DMD or the imaging sensor?

Fig. 1

Concept for hardening a sensor against laser dazzling using a DMD: (a) operation mode for regular imaging, (b) operation way with high attenuation of dazzling light amplification by stimulated emission of radiation light, and (c) photo of the hardened sensor.

From an economic betoken of view, 1 would prefer that the imaging sensor is damaged first, because the imaging sensor is quite inexpensive compared to the DMD. The user of such a system would prefer the DMD to be damaged start. In this case, the DMD acts every bit a sacrificial element for the imaging sensor. The organization could still be used, only some distortions would occur (such as a color baloney or contrast loss) in the sensor image.^vi The magnitude of the distortion would depend on the size of the damage on the DMD.

Of class, in that location are some publications regarding laser harm thresholds of photographic camera sensors and DMDs. For case, Becker et al.^{7 , viii} studied laser-induced damage of CCD sensors by nanosecond light amplification by stimulated emission of radiation pulses at 1064 nm. Start functional changes were observed at a fluence value of $0.55\mathrm{J}/{\mathrm{cm}}^{\mathrm{two}}$ . Guo et al.⁹ investigated the damage effect on CMOS detectors irradiated by laser pulses at 1064 nm. In the example of nanosecond laser pulses, the damage threshold was around $0.38\mathrm{J}/{\mathrm{cm}}^2$ and in the case of picosecond laser pulses, they received a threshold value of $0.02\mathrm{J}/{\mathrm{cm}}^{\mathrm{ii}}$ .

For the DMD, the maximum power density for homogeneous illumination is specified as $25\mathrm{W}/{\mathrm{cm}}^{\mathrm{ii}}$ in the visible spectral range.^{10 , 11} However, in a publication of Faustov et al.,¹² a damage threshold higher up 22 mW for a laser wavelength of 633 nm is reported for the case in which the laser light is focused onto a single micromirror ( $13.7\times 13.7\mu {\mathrm{m}}^2$ ). This value corresponds to an irradiance of $\sim 12\mathrm{kW}/{\mathrm{cm}}^2$ and is far above the value stated by Texas Instruments. This discrepancy motivated us to perform light amplification by stimulated emission of radiation impairment experiments with a DMD as well as standard CCD and CMOS cameras.

In this work, we examine the laser damage threshold and the morphology of laser damage in the photographic camera images of CMOS and CCD devices. The surface of the DMD is investigated by means of the image of a CMOS device. We volition return to that later. The results enable us to get first indications, determine which of these devices suffer damage first from light amplification by stimulated emission of radiation radiations, and to bank check if the DMD acts as a sacrificial chemical element in the optical setup.

Detailed knowledge about light amplification by stimulated emission of radiation damage thresholds of imaging sensors is not only important for scientists working in the area of laser protection. Information technology is besides of involvement, for example, to the manufacturers of devices for laser beam label. Photographic camera-based light amplification by stimulated emission of radiation beam profilers or $M^{\mathrm{two}}$ measurement systems employ imaging sensors that are direct exposed to a laser beam. They can suffer damage when the laser intensity exceeds the impairment threshold.

When using different laser configurations, nosotros expect unlike harm mechanisms to affect the devices.¹³ In the example of irradiation with millisecond- and longer-light amplification by stimulated emission of radiation pulses, thermal effects that convert the laser energy into thermal energy are the main damage mechanism. This is related to the melting or vaporization of the textile. On the contrary, irradiation with shorter laser pulses calls other damage mechanisms into play. That applies especially for highly transmitting materials, where the impairment threshold is rather high. Laser pulses on the order of nanoseconds or picoseconds cause electrically induced impairment due to the corresponding high laser electrical field intensity and the short elapsing of the laser pulse effects, such as dielectric breakdown. During such a breakdown mechanism, the insulator becomes electrically conductive plasma which causes thermal harm in a next step also as shock damage and mechanical harm. For further details please refer to Ref. 14. Nosotros expect, that the disturbance in the output images of the cameras is unlike for the continuous-moving ridge (CW)-laser and the pulsed laser.

2.

Setup and Procedures of the Experiment

2.1.

Instrumental Setup

The schematic of the experiment is shown in Fig. 2. The sensors under examination were irradiated by two different types of light amplification by stimulated emission of radiation systems to perform the light amplification by stimulated emission of radiation-induced harm tests:

• • Pulsed laser organization: Nd:YAG light amplification by stimulated emission of radiation, operating at a wavelength of 1064 nm with a maximum pulse energy of 300 mJ at a repetition rate of x Hz and a spatial Gaussian free energy distribution (InnoLas Spitlight Hybrid). Nosotros used the second harmonic at a wavelength of 532 nm, generated by a harmonic Generating Assemblies containing kaliumtitanylphosphate crystals. The temporal pulse length was 10 ns and the axle diameter was six mm ( $1/{\mathrm{eastward}}^2$ ).

• • CW-laser system: Diode-pumped solid-state light amplification by stimulated emission of radiation (DPSS, Laser Quantum Ventus 532) with a wavelength of 532 nm and a axle size of 1.5 mm ( $1/{\mathrm{eastward}}^2$ ). The bachelor light amplification by stimulated emission of radiation ability exceeded 500 mW.

Fig. 2

Experimental setup: (a) configuration for CCD/CMOS damaging testing and (b) configuration for the DMD testing.

The incident light amplification by stimulated emission of radiation energy of the pulsed laser was adjusted by varying the time delay between the offset of the wink lamp pulse and the trigger of the Pockels cell. The power level of the CW-laser source was controlled by changing the current of the diode driver from 49% up to 100%.

Furthermore, we used a set of neutral density filters with different optical densities (ODs) ranging from OD 0.v to OD 3.0. To command different exposure times of the CW-light amplification by stimulated emission of radiation source, nosotros used a laser shutter (Uniblitz Shutter Systems VS25). Finally, the laser beam was focused by a lens (Apo-Rodagon Due north 4.0/80, Qioptiq) with a focal length of $f=\mathrm{eighty}\mathrm{mm}$ and an aperture of $f/5.6$ , thus the beam bore in the focal plane was measured by the beam profiler BP209 from Thorlabs using a scanning slit method. The measurement with the beam profiler took place before the actual experiment. Therefore, we were able to decide the diameter ( $\mathrm{ane}/{\mathrm{eastward}}^2$ ) of the laser spot in the focal airplane and got a value of $2{\omega }_1=25.7\mu \mathrm{one\; thousand}$ for the CW-laser source and $2{\omega }_2=\mathrm{28.ii}\mu \mathrm{m}$ for the pulsed laser source. The effective diameter $d_{\mathrm{eff}}$ of a uniform cylindrical axle with the same peak intensity and total power as a cylindrical Gaussian axle is¹⁵

Eq. (i)

$$d_{\mathrm{eff}}=\sqrt{2}\omega ,$$

and the associated effective spot size $A_{\mathrm{eff}}$

Eq. (2)

$$A_{\mathrm{eff}}=\frac{\pi {\omega }^2}{2}.$$

According to this, the effective spot size in the focal plane is $A_{\mathrm{eff}}=3.12\times {\mathrm{ten}}^{-\mathrm{vi}}{\mathrm{cm}}^2$ for the pulsed laser and $A_{\mathrm{eff}}=\mathrm{ii.59}\times {10}^{-6}{\mathrm{cm}}^2$ for the CW-laser.

The device under test was installed on a 3-D translation phase, so that we could shift the sensor into the focal plane of the lens and each pulse could be exposed to an unused test site (Fig. 2). To find whatever changes in the image of the camera, we observed the output indicate of the camera on the estimator. The experimental setup for the investigation concerning the DMD differed from those concerning the CMOS/CCD sensors. The DMD was as well put in the focal aeroplane, only in contrast to the experiments concerning the CMOS/CCD cameras, we used an observer photographic camera [camera 2 in Fig. ii(b), VRmagic VRmFC-22/BW] exterior the optical axis to record the position of the laser spot on the DMD and a second observing camera [camera 1 in Fig. 2(b), The Imaging Source DFK22AUC03], which could exist flipped into the optical axis, and then we could take a picture of the DMD earlier and after each laser shot to detect whether harm occurred or not.

To compare the output images of the cameras before and after each shot, we illuminated the cameras equally uniformly as possible. For that purpose, we used an integrating sphere (connected to a stabilized fiber-coupled calorie-free source SLS201/M from Thorlabs), which could be flipped into the axle path in front of the lens. In the case of the DMD sensor, nosotros illuminated the sensor from outside the optical axis with a halogen cold calorie-free source (Schott KL 255 LCD) so that the center of the DMD is illuminated as uniformly every bit possible. Note that the micromirrors' normal and the optical axes course an angle of $+12\mathrm{deg}$ to the beam centrality in the "on" position and an angle of $-12\mathrm{deg}$ in the "off" position with the result that in one position the light was reflected to observing photographic camera 1 and in one it was non.

The experiment described was performed using monochromatic and colour CMOS cameras including an imaging sensor Aptina MT9V024 with a resolution ( $\text{horizontal}\times \text{vertical}$ ) of $744\times 488\text{pixels}$ and a pixel size of $\mathrm{half-dozen}\mu \mathrm{m}\times \mathrm{half\; dozen}\mu \mathrm{grand}$ . We too examined the impairment formation of monochromatic and color CCD cameras operating with an imaging sensor (monochromatic: Sony ICX098BQ, colour: Sony ICX098BL) at a resolution of $640\times 480\text{pixels}$ and a pixel size of $\mathrm{5.viii}\mu \mathrm{m}\times \mathrm{five.8}\mu \mathrm{grand}$ . The 3rd device under examination was a DMD with $1024\times 768$ micromirrors (Texas Instruments DLP7000). All the investigated samples were encapsulated with a protecting glass plate, so we did not expect any contamination (e.1000., dust particles) directly on the blank imaging sensor. It can, therefore, be considered that there is no influence on the measured damage thresholds by contagion. Of course, impurities in the starting materials used in sensor production cannot be excluded. The distance from the surface of the cover glass to the surface of the imaging sensor was about $1.94\pm 0.15\mathrm{mm}$ in case of the CMOS and CCD cameras and the distance from the surface of the cover glass to the surface of the mirror array was about $2.9\pm 0.1\mathrm{mm}$ in case of the DMD. The distance corresponded approximately to the double of the Rayleigh range. This means that the light amplification by stimulated emission of radiation power density or the laser pulse free energy density on the glass surface was significantly lower than the ane on the surface of the imaging sensor. All samples are listed in Table i.

Table 1

Specifications of the samples under test.

Exam sample	The Imaging Source DFK21AU04 (color)	The Imaging Source DMK21AU04 (monochromatic)	The Imaging Source DFK22AUC03 (color)	The Imaging Source DMK22AUC03 (monochromatic)	Texas Instruments DLP Discovery 4100 Evolution Kit
Device	Sony ICX098BQ	Sony ICX098BL	Aptina MT9V024	Aptina MT9V024	Texas Instruments DLP7000
Device type	CCD	CCD	CMOS	CMOS	DMD
Format (in.)	i/iv	1/4	1/iii	one/3	0.7
Resolution ( $\mathrm{H}\times \mathrm{5}$ ) (px)	$640\times 480$	$640\times 480$	$744\times 480$	$744\times 480$	$1024\times 768$
Pixel size ( $\mathrm{H}\times \mathrm{Five}$ ) ( $\mu {\mathrm{m}}^2$ )	$\mathrm{five.6}\times \mathrm{5.vi}$	$5.6\times \mathrm{v.6}$	$\mathrm{six}\times 6$	$6\times \mathrm{six}$	$13.68\times 13.68$
Chip depth (flake)	viii	viii	8	8	—
Sensitivity (lx)	0.ane	0.03	five	0.1	—
IR cut filter	Aye	No	Yes	No	—
Shutter	Global	Global	Global	Global	—
Exposure time used in the experiment (ms)	120	120	188	188	—

2.2.

Implementation of the Experiment

The experiments did not take identify in a classified clean room. Therefore, we had to ensure that the protecting glass plates of the imaging sensors were clean from harmful dust and dirt earlier each test procedure. The test site of the investigated object was positioned in the focal plane. To measure the impairment threshold, we used the 1-on-one test style, and then each test site was irradiated by 1 pulse or in the case of CW-laser radiations for a certain exposure time (0.25, ane, 5, and ten s). Before each light amplification by stimulated emission of radiation shot, we took 2 reference images: an illuminated image (flat-field) and a night-frame with the same exposure time every bit nosotros used taking the raw paradigm. In the following, we exposed the sensor with a highly attenuated laser beam, so that no damage could occur to the sensor. We captured a picture during this laser irradiation with the effect that nosotros could make up one's mind the positon of the affect of the laser beam on the sensor. After the irradiation with high-power/pulse free energy, we took an unexposed epitome (dark image) and an illuminated prototype (bright image). Finally, we changed the test site as whether the sample was damaged or non and repeated the whole procedure after increasing the light amplification by stimulated emission of radiation power/pulse energy. Figures three and 4 show a selection of raw images of the inflicted damage with and without illumination.

Fig. three

Raw images of various monochrome and colour cameras showing pulsed laser-induced damages in the (a) dark images and (b) in the bright images. The laser fluence used to cause damage increases from height to bottom.

Fig. iv

Raw images of various monochrome and color cameras and the DMD showing pulsed laser-induced damages in the bright images. The light amplification by stimulated emission of radiation power density used to crusade impairment increases from left to right.

Fig. 5

Steps of image processing in instance of an illuminated camera image (bright image) and an unexposed camera image (dark image).

3.

Data Analysis and Results

3.ane.

Paradigm Processing

In the instance of pulsed laser irradiation, the damaged expanse in dark images and bright images appeared in the aforementioned shape. For the sake of simplicity, we used the night images to evaluate the damage threshold. We cutting the corresponding expanse kickoff and then subtracted each prototype with a reference dark-frame (see Fig. five). In the case of CW-laser irradiation, we used the vivid images for evaluation, considering there was no damage visible in the nighttime images. We corrected each raw epitome by apartment-field correction. This was done by removing the DC showtime signal by subtracting the dark-frame from the raw image and multiplying information technology with the normalized flat-field correction image.^sixteen Color images were converted into grayscale. Finally, we divers damage as a ten% divergence from intensity of the normalized and scaled reference epitome. Based on this threshold, we converted the postcorrection prototype into a binary image, and then that all pixel values below the threshold value are set to 0 and those above this value to ane. Afterward, we drew a profile line around all pixels with value i and, finally, we calculated the expanse inside this contour.

As long equally the laser damage resembles a circular disc, this method is quite easy to apply, meet Fig. v(c). Still, for larger laser powers/pulse energies, some deviations can occur:

• • Line damage: This means that complete columns or rows of pixels corresponding to the position of the laser spot accept failed. Line damage can occur in 2 ways: only columns or rows neglect, meet Fig. five(b), or columns and rows fail, see Fig. 5(a).

• • The damage design can exist star shaped [run into Fig. five(a)].

In the case of unmarried line damage, the profile line is a convex hull around all pixels with the value 1 [see yellow line in Fig. 5(b)]. In the example of star-shaped impairment, including line harm, the damaged surface area was besides marked by contour lines. Additionally, the mean distance from the position of the laser spot eye on the surface to each outer edge correct beside the line harm was adamant. Later on that, a mean circle was computed into the epitome [run across thin red line in Fig. 5(a)]. We defined the damaged surface area as all pixels that were independent either within the contour or inside this mean circle.

3.2.

Interpretation of the Damage Threshold Based on Thermal-Induced Amercement

A logarithmic relationship between the damaged area and laser pulse energy is used to determine the impairment threshold and is described in various other experiments.^{17 – 20} In their work, the authors observed by microscope images the development of a series of concentric rings, generated through the exposure of ultrashort laser pulses, which they chosen a specific amorphous band pattern. Apparently, the ring formation is associated with photo-thermal damage of the subject due to laser radiation. The spatial intensity distribution of a Gaussian laser beam is described as

Eq. (iii)

$$\mathrm{\varphi }(r)={\mathrm{\varphi }}_0{e}^{-\mathrm{two}\frac{r^2}{{\omega }_0^2}}.$$

Knowing that the intensity at the rim of the damaged surface area $i$ with altitude $r_i$ from the center of the laser spot corresponds to the impairment threshold ${\mathrm{\varphi }}_{\mathrm{th}}$ , nosotros go

Eq. (four)

$${\mathrm{r}}_i^2=\frac{{\omega }_0^2}{2\pi }\ln \left[\frac{\mathrm{\varphi }}{{\mathrm{\varphi }}_{\mathrm{thursday}}}\right].$$

In the case of pulsed lasers, the laser-induced damage threshold (LIDT) of arresting materials is a constant value if measured in terms of $\mathrm{J}/{\mathrm{cm}}^{-2}$ so $\mathrm{\varphi }$ corresponds to the applied laser fluence $F$ , and in the example of CW-laser radiation, the LIDT is a abiding value if measured in $\mathrm{Westward}/{\mathrm{cm}}^{-\mathrm{ii}}$ so $\mathrm{\varphi }$ corresponds to the irradiance. The distance $r_i$ is the outer radius of the damaged surface area.²¹ The human relationship of Eq. (iv) likewise applies in the instance of the ablation caused by nanosecond pulses.²² Following this idea, we plotted the area of the damaged surface of the detector cloth in pixels versus the fluence or the power density in a semilog plot, respectively, and fitted direct lines to the data co-ordinate to Eq. (4). The merely departure from other experiments is that we did not look at the physically damaged area of the detector material. Moreover, we were interested in the light amplification by stimulated emission of radiation-induced impairment, which became visible in the output camera image. We defined the "reconstructed axle bore" (RBD) every bit the beam diameter we got from the slope of Eq. (four). It represents the required beam diameter of the laser source on the surface of the examination object, if the expansion of disturbance in the camera image resembled the concrete harm in the sensor.

For pulsed light amplification by stimulated emission of radiation radiation, nosotros identified four different types of light amplification by stimulated emission of radiation-induced impact on the cameras, which nosotros classified in iv groups (run into Table 2). Each group was marked by vertical blue dashed lines in the graphs of Fig. 6Fig. 7–8. In the example of CW-radiation, only spot damage and in a few occasions line harm occurred.

Table two

Groups of laser-induced impact on the devices

Grouping	Clarification
I	No harm is observed
II	Spot harm occurs (CMOS and CCD camera)
III	Spot damage and line harm occurs (CMOS photographic camera), spot impairment elongates in vertical direction and finally transfers into full line damage (CCD camera)
Four	Star-shaped spot damage including full line damage (CMOS camera)

Fig. 6

Damaged area size of a monochrome CMOS photographic camera as a role of pulsed laser fluence in semilog plot: (a) total data and (b) section of the total information consisting impairment types I and II. The vertical blue dashed lines demarcate different stages of damage phenomenon.

Fig. 7

Size of the damaged area of a colour CMOS camera equally a part of pulsed laser fluence in semilog plot: (a) full information and (b) section of the total information consisting harm types I and Two. The vertical blue dashed lines demarcate different stages of impairment miracle.

Fig. viii

Size of the damaged area of a color CCD camera every bit a function of pulsed laser fluence in a semilog plot. The blue dashed lines demarcate different stages of damage phenomenon.

iii.3.

Observation, Morphology, and Threshold of Pulsed Laser-Induced Damage on CMOS Cameras

In the instance of the monochrome CMOS cameras exposed to pulsed light amplification by stimulated emission of radiation light, first sensor damages were observed at a fluence of $0.1\mathrm{J}/{\mathrm{cm}}^{\mathrm{two}}$ . No harm was observed below a level of $0.043\mathrm{J}/{\mathrm{cm}}^2$ . The amercement appear as white "hot pixels" in the dark and in the vivid images [see Fig. 3(1–3)]. This may indicate that pulsed light amplification by stimulated emission of radiation radiation causes a change in the bandgap of the semiconductor or a change in the insulation area resulting in an increasing leakage current. Whether there is incident low-cal or not, white pixels are quite credible. The shape of the harm was mainly round and slightly elongated in the horizontal direction. At further increased fluence, the spot damage started to develop star-shaped edges around a circular center, starting from a value of $14.9\mathrm{J}/{\mathrm{cm}}^{\mathrm{two}}$ . In Fig. three([3b]), such peaks tin also be recognized at fluence levels at which line damage occurs, but not to a great extent. Line damage, an emergence indicating that a whole line in the horizontal and/or in vertical direction became inoperative, started at a value of about $1.42\mathrm{J}/{\mathrm{cm}}^{\mathrm{ii}}$ . In the vertical direction, the line impairment extended steadily to the borders of the detector. At high fluence values, the line damage on the right-hand side of the damaged expanse was stronger than the line impairment on the left-hand side. Line harm may be caused by signal interruption because of the device excursion fuses being cut. There was no visible difference in the shape of the damaged expanse for the bright images and the dark-frame. The graph in Fig. 6(a) shows the damaged area as a function of pulsed laser fluence for the monochrome CMOS camera. We performed ii linear fits [cf. Eq. (iv)] to the data with different slopes. For the fit concerning the data of groups I and II [see the red line in Fig. half dozen(b)], we estimated a impairment threshold of $F_{\mathrm{th}}=(0.076\pm 0.019)\mathrm{J}/{\mathrm{cm}}^{\mathrm{two}}$ and a respective RBD of $2{\omega }_0=(\mathrm{xviii.9}\pm \mathrm{3.viii})\mu \mathrm{g}$ . This estimated value corresponds to the previously measured beam bore, so nosotros can assume that the physical damage of the sensor and the disturbance in the corresponding output epitome are comparable. As shortly as the spot impairment starts to develop as star shaped, the linear fit to the data becomes a different slope [see greenish line in Fig. 4(a)], which would result in another RBD of $\mathrm{two}{\omega }_0=(1285\pm 5)\mu \mathrm{m}$ . Consequently, we can conclude that the disturbance in the output prototype of the photographic camera grows faster than the physical harm on the sensor scrap. We interpreted the intersection of the green and the cherry-red line as the commencement of the formation of the star-shaped character at a fluence value of $F_s=47.2\mathrm{J}/{\mathrm{cm}}^2$ .

The formation of impairment by irradiating color CMOS cameras started to abound at a level of $F=0.053\mathrm{J}/{\mathrm{cm}}^2$ . No damage was observed beneath $0.043\mathrm{J}/{\mathrm{cm}}^2$ . Line damage was observed at a level of $F=3.7\mathrm{J}/{\mathrm{cm}}^{\mathrm{ii}}$ . Star-shaping started at a value of $\mathrm{38.half-dozen}\mathrm{J}/{\mathrm{cm}}^{\mathrm{ii}}$ . The shape of the spot damage was also round and slightly elongated in the horizontal management. No departure in the composition of the damage in night and the brilliant images was observed. It is hitting that the damaged pixels appeared predominantly dark-green, both in the dark epitome and in the bright image [see Fig. 3(4–5)]. Merely in the example of higher pulse energies, did the damaged pixels announced mainly white in the center of the damaged area and green at the edges of the area in the dark image. Information technology is remarkable that in the apartment-field image, the edges of the damaged area appeared in red [meet Fig. iii(6)]. The line harm represented itself in red, yellow, and black lines or in blue and black lines for lower energies. Only in case of higher energy, was there a bunch of black lines with blue lines at one outer edge and reddish likewise as xanthous lines at the other border. The photographic camera sensor was completely destroyed in the sense that the output prototype no longer reacted to incident low-cal, at a level of $\mathrm{ii.nine}\mathrm{kJ}/{\mathrm{cm}}^2$ . The red curve in Fig. 7 represents the dependence of the damaged area versus input fluence for low light amplification by stimulated emission of radiation pulse free energy. From the linear fit, we guess a damage threshold of $F_{\mathrm{th}}=(0.035\pm 0.009)\mathrm{J}/{\mathrm{cm}}^2$ and an RBD of $2{\omega }_0=(14.9\pm 4.5)\mu \mathrm{k}$ . The intersection of the green and the cerise curve lies at a fluence value of $F_s=102\mathrm{J}/{\mathrm{cm}}^{\mathrm{ii}}$ . The fact that the impairment threshold of the color CMOS camera was lower than the impairment threshold of the monochromatic device is an indication that the first damage in color cameras emerges in the Bayer filter.

iii.4.

Observation, Morphology, and Threshold of Pulsed Laser-Induced Damage on CCD Cameras

For monochrome CCD cameras, the germination of harm started at a fluence of $F=0.032\mathrm{J}/{\mathrm{cm}}^2$ . No damage was observed below a level of $0.004\mathrm{J}/{\mathrm{cm}}^2$ . Line damage was too observed and started at a fluence value of $F=0.35\mathrm{J}/{\mathrm{cm}}^2$ . At a fluence value of $F=147\mathrm{J}/{\mathrm{cm}}^{\mathrm{two}}$ , the whole sensor was broken. The damage appeared as a white "hot pixel" in the dark image besides every bit in the bright paradigm [Fig. 3(seven–9)]. Line damage evolved only in the vertical direction. The damage on the camera started as point harm and, in contrast to CMOS cameras, the damage elongated in the vertical management as the free energy increased, starting at a fluence of $F=0.14\mathrm{J}/{\mathrm{cm}}^{\mathrm{two}}$ . This behavior was as well observed in other works.²³ For most CCD cameras, the electrodes in each pixel are bundled in such a mode that the charge is transferred in the vertical direction along the cavalcade to the last row (readout register). To avoid the charges escaping laterally, there are some "aqueduct-stops" implanted nigh to the surface to isolate the charge packets from side by side columns. In the case of strong irradiation, the created accuse carriers prefer the vertical direction. The shape of the damage was equal in both the night-frame and apartment-field images. Due to the destruction of the sensor, nosotros did not receive enough data to perform linear bend-fitting.

In the case of the color CCD camera, at start glance the bright image exhibited a unlike red level baseline after each shot. The germination of spot damage started at a level of $F=0.034\mathrm{J}/{\mathrm{cm}}^{\mathrm{two}}$ and no impairment was observed beneath a fluence value of $0.014\mathrm{J}/{\mathrm{cm}}^2$ . Line damage occurred at a level of $F=0.49\mathrm{J}/{\mathrm{cm}}^{\mathrm{ii}}$ and appeared scarlet, blue, or yellowish. At a level of $3.16\mathrm{J}/{\mathrm{cm}}^2$ , the camera was destroyed. The damage occurred nearly circularly above a fluence of $0.064\mathrm{J}/{\mathrm{cm}}^2$ and appeared green in the night image and yellow in the bright paradigm. The damage exhibited the same shape in the bright and the dark images. Due to the red background in the image, the damaged pixels, which are related to the green channel of the sensor, appeared in yellow. The damage shape elongated in the vertical direction with increasing free energy. At higher laser free energy, the color of the damaged area turned mostly white, but at the outer edge of the harm, all colors are represented. From the fit in Fig. eight, we got a damage threshold $F_{\mathrm{th}}=(0.041\pm 0.003)\mathrm{J}/{\mathrm{cm}}^{\mathrm{two}}$ and an RBD of $2{\omega }_0=(37\pm 5)\mu \mathrm{chiliad}$ .

3.v.

Observation, Morphology, and Threshold of CW-Light amplification by stimulated emission of radiation-Induced Damage on CMOS Cameras

In contrast to pulsed laser-induced damage, no visible damage occurs in the dark prototype. Therefore, only the vivid images were used to analyze the visible damage. Showtime impairment occurred in instance of the monochrome CMOS cameras for exposure times of 0.25, 1, 5, and ten s at a power density of 85, 85, 57, and $49\mathrm{kW}/{\mathrm{cm}}^2$ , respectively. The shape of the impairment was more often than not circular and slightly blurred, because the damaged pixel became less sensitive but did not fail completely. The impairment appeared dark in the flat-field image in opposition to the pulsed-laser damage, where the impairment appeared white in the flat-field image. Nosotros also observed line impairment starting from a power density of $196\mathrm{kW}/{\mathrm{cm}}^2$ . From the fit in Fig. 9(a), we got a damage threshold depending on the exposure time of $F_{\mathrm{th}}=[75\pm \mathrm{vii},73\pm 13,56\pm 4,48\pm 3]\mathrm{kW}/{\mathrm{cm}}^{\mathrm{ii}}$ . Evidently, the slope of the lines in crimson and black are dissimilar from those in green and blue. According to Eq. (iv), dissimilar slopes are associated with different RBD $2{\omega }_0=[\mathrm{xiv}\pm \mathrm{v},16\pm \mathrm{iv},20\pm 3,21\pm \mathrm{iii}]\mu \mathrm{one\; thousand}$ .

Fig. 9

Size of the damaged area of a CMOS camera versus power density of a CW-light amplification by stimulated emission of radiation in a semilog plot: (a) monochrome camera and (b) color camera.

In the instance of the color CMOS cameras, impairment started at a ability density of $46\mathrm{kW}/{\mathrm{cm}}^{\mathrm{ii}}$ for an exposure time of 0.25 s. No line impairment was observed. Damaged pixels seemed about royal, in other words, a combination of blue and carmine pixel values. Just as it was in the example of damage to the monochromatic device, the shape appeared almost round and blurred. From the fit in Fig. 9(b), we got a damage threshold of $F_{\mathrm{th}}=(56.7\pm \mathrm{one.8})\mathrm{kW}/{\mathrm{cm}}^2$ and an RBD $2{\omega }_0=(12.6\pm \mathrm{0.v})\mu \mathrm{m}$ .

3.6.

Observation, Morphology, and Threshold of CW-Laser-Induced Damage on CCD Cameras

For the monochrome CCD camera, it was quite challenging to cause damage to the sensor. Damage started to occur for exposure times of 0.25, 1, v, and 10 south at power densities of 163, 139, 139, and $139\mathrm{kW}/{\mathrm{cm}}^2$ , respectively. No impairment occurred beneath a value of $135\mathrm{kW}/{\mathrm{cm}}^2$ for exposure times of 1, v, and 10 southward and below a value of $159\mathrm{kW}/{\mathrm{cm}}^2$ for an exposure time of 0.25 southward. The shape of harm is nearly circular and the damaged pixels are dark in the output prototype. From the fit in Fig. 10(a), nosotros got a damage threshold of $F_{\mathrm{th}}=[146\pm 9,118\pm 9,93\pm \mathrm{xix},95\pm 23]\mathrm{kW}/{\mathrm{cm}}^{\mathrm{ii}}$ and an RBD $\mathrm{two}{\omega }_0=[22\pm \mathrm{iii},27\pm 3,25\pm \mathrm{iii},25\pm 3]\mu \mathrm{thou}$ , respectively.

Fig. 10

Size of the damaged area of a CCD camera versus ability density of a CW-light amplification by stimulated emission of radiation in a semilog plot: (a) monochrome camera and (b) color camera.

Fig. 11

Size of the damaged area of the DMD versus power density of a CW-light amplification by stimulated emission of radiation source.

In case of the color CCD camera, we observed the same behavior as in example of the pulsed-laser-induced damage to the photographic camera of the same type. The bright image exhibited a different carmine baseline afterward each shot. First impairment for exposure times of 0.25, 1, v, and 10 s started at a power density of 16, xvi, 9, and $9\mathrm{kW}/{\mathrm{cm}}^{\mathrm{ii}}$ and no damage occurred beneath a level of 9.two, 9.two, 5.5, and $5.5\mathrm{kW}/{\mathrm{cm}}^2$ . No line damage was observed. The impairment shape was circular. Damaged pixels were almost purple or deep carmine because of the loftier red levels. The data of the green pixels was reduced, but they were not completely insensitive. From the fit in Fig. 10(b), we estimated a damage threshold of $F_{\mathrm{th}}=[14\pm \mathrm{two},13\pm 2,11\pm \mathrm{one},\mathrm{viii.one}\pm \mathrm{0.eight}]\mathrm{kW}/{\mathrm{cm}}^2$ and an RBD of $\mathrm{two}{\omega }_0=[\mathrm{18.five}\pm 3.6,\mathrm{xviii.5}\pm \mathrm{iii.vi},\mathrm{xviii.6}\pm 3.6,17.5\pm 3.8]\mu \mathrm{yard}$ .

3.7.

Ascertainment, Morphology, and Threshold of CW-Laser-Induced Damage on Digital Micromirror Device

Initially, damage appeared every bit pixels with reduced intensity if the DMD was illuminated. Due to the fact that ane micromirror was represented past 1.5 pixel in the camera prototype, there are always areas which are a combination of damaged mirrors and undamaged ones. Light amplification by stimulated emission of radiation-induced impairment to the micromirrors could cause a decrease in reflection or could destroy the tilt mechanism. Damage on the DMD started at a ability density of $19.3\mathrm{kW}/{\mathrm{cm}}^2$ for an exposure time of 0.25 south. No harm was observed below a power density of $\mathrm{1.ix}\mathrm{kW}/{\mathrm{cm}}^2$ . The damage on the DMD sensor is virtually circular. The fit (red slope in Fig. 11) from Eq. (iv) led to a damage threshold of $(\mathrm{21.ix}\pm \mathrm{i.two})\mathrm{kW}/{\mathrm{cm}}^{\mathrm{ii}}$ and an RBD of $2{\omega }_0=(15\pm 4)\mu \mathrm{yard}$ .

4.

Decision

We accept studied the formation and development of laser-induced damage to CMOS and CCD cameras by ways of pulsed and CW-light amplification by stimulated emission of radiation radiation. The results for pulsed laser radiation are listed in Tabular array 3. The harm was observed in both the brilliant images and the dark images. Information technology is worth mentioning that starting from a defined value of laser fluence the impairment on CMOS evolved as star shaped and on CCD damage elongated in the vertical direction. The color cameras exhibited the everyman damage threshold. Generally, the harm threshold of the CCD cameras was lower than that of the i of the CMOS cameras.

Table iii

Results from the 1-on-1 examination for pulsed laser sources.

Test sample	Impairment threshold experimental information (J/cm2)	Damage threshold fit to data (J/cm2)	Line-damage threshold (J/cm2)	Star-shapea or vertical elongatedb threshold (J/cm2)
CMOS
Mono	0.099	$0.08\pm 0.02$	14.ix	47.two
Colour	0.053	$0.035\pm 0.01$	38.6	102
CCD
Mono	0.032	—	0.35	0.fourteen
Color	0.034	$0.041\pm 0.003$	0.49	—

We also examined the germination and development of CW-laser-induced harm to CMOS and CCD cameras on the i hand and for the DMD (come across Table 4) on the other hand. Damage merely manifested itself in the brilliant image. Harm to the color devices occurred before than damage to the monochromatic cameras. The harm threshold of the DMD lies in the magnitude of the color CCD camera, only significantly beneath those of the other devices. For the latter sensors, a DMD could be installed as a sacrificial element in front end of the sensor. Further investigations should contain pulsed laser-induced damage to the DMD with pulse lengths on the order of nanoseconds and picoseconds (also to the CMOS and DMD). Additionally, nosotros volition investigate the physical damage on the sensor, which is visible nether the microscope.

Table 4

Results from the 1-on-1 examination for CW-light amplification by stimulated emission of radiation sources.

Test sample	Damage threshold experimental data (kW/cm2)				Damage threshold fit (kW/cm2)
Exposure time (s)	0.25	ane	5	10	0.25	ane	5	10
CMOS
Mono	85	85	57	49	$75\pm 7$	$73\pm 15$	$56\pm \mathrm{iv}$	$48\pm 3$
Colour	46	—	—	—	$56.7\pm \mathrm{1.eight}$	—	—	—
CCD
Mono	163	139	139	139	$146\pm 9$	$118\pm 9$	$93\pm \mathrm{xix}$	$95\pm 21$
Color	16	16	9	9	$\mathrm{fourteen}\pm 2$	$13\pm \mathrm{ii}$	$11\pm 1$	$\mathrm{viii.one}\pm 0.8$
DMD	19.3	—	—	—	$\mathrm{21.nine}\pm 1.2$	—	—	—

References

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" DLP^® 0.seven XGA 2xLVDS type A DMD ," DLPS026A Datasheet, (2012). Google Scholar

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" Laser power treatment for DMDs ," DLPA027 Datasheet, (2012). Google Scholar

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D. Ristau, Light amplification by stimulated emission of radiation-Induced Damage in Optical Materials, CRC Press/Taylor & Francis Group, New York (2015). Google Scholar

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A. E. Siegman, Lasers, 663 –667 Oxford University Press, University Scientific discipline Books, Oxford, England (1986). Google Scholar

Biography

Bastian Schwarz has been a research scientist at Fraunhofer IOSB, Ettlingen, Deutschland, since 2013. He graduated in physics from the Academy of Freiburg in 2012 and worked at the Kiepenheuer Constitute for Solar Physics. Since 2013, his research areas include light amplification by stimulated emission of radiation protection and laser damage performance.

Gunnar Ritt is a research acquaintance at Fraunhofer IOSB, Ettlingen, Germany. He received his diploma and PhD degrees in physics from the University of Tübingen, Federal republic of germany, in 1999 and 2007, respectively. His primary research focus is on laser protection.

Michael Koerber has been a enquiry scientist at Fraunhofer IOSB, Ettlingen, Germany, since 2013. He is part of the optical countermeasure and laser protection group and participated in several projects. He received his main of science degree in physics from the University of Konstanz in 2012. He works in the field of laser spectroscopy, nonlinear optics, femtosecond optics, and optical countermeasures.

Bernd Eberle is a senior scientist at Fraunhofer IOSB in Ettlingen, Germany, where he is head of the optical countermeasure and laser protection group. He received his diploma degree in physics at the University of Konstanz in 1983. He received his PhD degree in physics at the University of Konstanz in 1987. His inquiry activities comprise laser technology, light amplification by stimulated emission of radiation spectroscopy, nonlinear eyes, femtosecond optics, optical countermeasures including protection confronting laser radiation and imaging laser sensors.

Source: https://www.spiedigitallibrary.org/journals/Optical-Engineering/volume-56/issue-03/034108/Laser-induced-damage-threshold-of-camera-sensors-and-micro-optoelectromechanical/10.1117/1.OE.56.3.034108.full?SSO=1

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