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Technical Course | Be Prepared for Something You Don’t Know about Speckles (Part II)

Last time, we have analyzed the misunderstandings of speckles, namely optical interference exists regardless of the light sources. Today, we plan to share some more about speckles with you.

About the existence of native and speckle-free laser imaging

Screenshot of the divided opinion in an article

Is native and speckle-free laser imaging a real thing? No!

Compared with the ordinary light sources that emit light in all directions, laser light source that is treated in a resonant cavity emits in only one direction. Meanwhile, as laser has good monochromaticity and narrow spectral width, laser light source generates speckles more easily than ordinary light sources do.

Color images displayed by a three-primary laser projector are the combination of red, green and blue lasers based on time and space distribution. Therefore, the phenomenon of speckles will inevitably be detected in laser projector. Lasers of all three colors will form speckles. However, visual effects vary as human eyes perceive light of different colors differently.

According to the curve of human vision, human eyes are sensitive to light of different colors to various extents. They are most sensitive to green light, followed by red and blue light. Thus, the impacts of speckles on human eyes vary with different colors. It is unscientific to say that the laser phosphor (blue laser + phosphor) is native and speckle-free. The reason why the combination of blue laser and phosphor "seemingly" has no obvious speckles is that human eyes are least sensitive to blue light, not that there are no speckles. Don't be misled.

Curve of human vision

Figure 1. Screenshot of the source text

In this part of the original article, the previous statement that

any light sources could result in speckles

is used as a premise to make an incorrect conclusion, that is,

there is no native and speckle-free laser imaging

However, as is analyzed in our previous article, if a light source is not coherent like a laser, it will not form speckles.

Phosphor produces typical incoherent light that is native and speckle-free! ! !

In addition, objectively speaking, although laser phosphor projection technology uses phosphor to produce green and red light (Why? Please refer to the cartoon below),

Figure 2. Schematic Diagram of Laser Phosphor Projection Technology

the blue light is still the laser. So it’s true that laser is part of the technology. The author calculated the proportion of laser in the laser phosphor projection. As is shown in Figure 3, to emit DCI-compliant white light of 60,000 lm, a RGB laser projector needs to emit a blue laser of 45.8 W, a green one of 81.0 W and a red one of 112.9 W, while a laser phosphor projector only needs to emit a blue laser of 46.7 W and phosphor light of 119.7 W. In other words, the coherent light emitted by a laser phosphor projector accounts for less than 1/5 of that emitted by an RGB one. Just like the original article says, there is no point in discussing toxicity without considering the dosage. Laser-induced speckles mainly affecting the visual experience is a visual psychological quantity. Human eyes have a resolution threshold. In other words, as long as the laser speckle contrast is below a certain threshold, the speckles are undetectable by viewers. At present, the establishment of standards for laser speckle contrast is underway. Here is a cool fact you may not know: compared with that of other laser projectors, the speckle contrast of laser phosphor projectors is the lowest pursuant to various standards! As a testament to the measurements against the standards, the operators of laser phosphor projectors do not have extra spending on eliminating the speckles for it is not needed. That’s also why it is fair to say laser phosphor projection is "native and speckle-free".

Figure 3. The power of a laser required for (a) an RGB laser projector and (b) that of a laser and phosphor for a laser phosphor projector to emit white light of 60,000 lm

In addition, phosphor is also effective in eliminating speckles. We have covered whether native and speckle-free laser imaging is a real thing. Let’s move on to the next divided opinion.

About the Safety of Speckles

Screenshot of the divided opinion in an article

Figure 4. Screenshot of the source text

Regarding the safety of speckles, some articles hold that since the light from the laser projector reflected on the screen is safe for human eyes, the speckles should be harmless too. The author admits that the light reflected on the screen is safe for human eyes. Even the original laser inside the laser projector, after passing through a series of optical devices and being evenly distributed over a wide area, is no longer dangerous narrow parallel beams of light from the projection lens. Therefore in 2015, the International Electrotechnical Commission (IEC) officially changed the classification of laser projectors from laser (IEC 50825-1: 2014) to light (IEC 62471-5: 2015)[4]

However, we should never neglect the glare hazard specified in the light standards. One of those articles also pointed out that "the power density at the place 0.25m to 2.5m away from the lens ranges from 1.97x103W/m2 to 213W/m2, which has exceeded the safety threshold for human eyes, and the light contacting with the eye within such distance should be avoided. At the place more than 2.5m away from the lens, the power density within a short period of time (0.1 to 1 second) is lower than the maximum allowable amount. No accidental damage caused by excessive irradiation may occur as human eyes will react timely to protect themselves. Therefore, such distance is safe for human eyes exposed to short-time irradiation." In other words, if you look directly at the projected light by accident, your eyes may be harmed. The closer you are to the lens, the stronger the luminous intensity will be, and the greater harm will do to the eyes. The question is will laser-induced speckles enhance the glare hazard? The answer is yes!

To further explain this, the author presents the process of research and calculation here for scientific discussion. If you find this part a bit long to read, please feel free to jump to the conclusion at the end.

When you look at the projected light that is scattered and coherent, a speckle pattern will be formed on the retinas of your eyes. In other words, there will be bright and dark speckles randomly distributed on the retinas. Below are the size and intensity of the speckles on the retinas.

According to the reference [5], the average size of speckles formed on a charge coupled device (CCD) with a lens is:

where f/# refers to the f-number of the lens, namely

where f refers to the focal length of the lens and the diameter of the lens aperture.

The structure of human eyes is similar to that of a CCD. The focal length f is 22.8mm [6], and the pupil diameter D is 3.2mm [7]. Therefore, for the 638 nm, 525 nm and 465 nm red, green and blue light, the average size of the speckles they form on the retina is 26.32 μm2, 17.82 μm2 and 13.98 μm2 respectively. In the fovea centralis, the average density of cone cells (photoreceptor cells) is 191,000 mm-2. In other words, the average size of a single pixel on the retina is 5.24 μm2. Therefore, a single speckle can cover 3 to 5 cone cells on average.

In laser-induced speckle imaging, the speckle intensity is a random quantity. The probability density function of speckle intensity can be obtained based on Goodman’s theory of Random Walk[3]. As a single speckle covers 3 to 5 cone cells on average, the probability density function of the intensity of the speckles on the retina is a negative exponential function[3] as shown in Figure 5. The function is as follows:

Where (I) refers to the average intensity of speckles, namely the luminous intensity of incoherent light of the same power reflected on the retina.

Figure 5. Histogram of the speckle intensity test results

The solid line represents a negative exponential function[3]

In practice, we care more about the probability of the speckle intensity exceeding a certain threshold:

According to formula (4), the probability that the intensity of speckles on certain photoreceptor cells on the retina exceeds 3 times the average intensity is 5%, and as for 9 times the average intensity, 0.01%. When the speckle intensity exceeds the safety limit, the speckles will cause harm to the photoreceptor cells there.

Take DCI-compliant white light as an example. When the light emitted by an RGB laser projector causes an average luminous intensity of I0 on the retina, the intensity of speckles distributed on different regions of the retina is shown in Figure 6 (a), from which we can tell that the luminous intensity on some cone cells is as high as 9I0. However, as most of the light beams emitted from a laser phosphor projector are incoherent (based on the calculation above, the coherent light beams is less than 1/5 of those emitted by the RGB projector), the luminous intensity on the cone cells is only 2I0. Therefore, compared with RGB projector, laser phosphor projector causes far less harm. I bet you don’t want to know how it feels to have glare as high as 9I0 hurting your eyes.

Figure 6 Intensity of speckles distributed on the retina generated by (a) RGB laser projector and (b) laser phosphor projector

This is the end of our discussion on some opinions about speckles raised by some articles. What I want to point out is that in the field of laser display, we need to take a positive attitude towards laser-induced speckles. Either to provide better visual experience or to reduce the risk of glare hazard, we should find a way to address the issue of speckles instead of ignoring it. Although there are many ways to eliminate speckles, the academic community is still trying to figure out more effective and stable measures that cost less. Here is an article on speckle elimination published this year[9]. If speckles are not an issue, why bother to eliminate them? In fact, phosphor is exactly an effective way of speckle elimination, which is used by ALPD® laser phosphor projection technology. Hopefully every player in the field of laser display could learn from each other, and contribute to the development of the entire industry.

Reference

[3]J. W. Goodman. Speckle Phenomena in Optics: Theory and Applications. 2006.

[4]https://www.lipainfo.org/laser-projection/laser-regulations/.

[5]S.Roelandt, et al.Standardized speckle measurement method matched to human speckle perception in laser projection systems. Optics Express. 2012, 20(8): 8770-8783.

[6] W. J. Smith. Modern Optical Engineering. New York: McGraw-Hill International Book Co, 1966.

[7]J. Pokorny and V. C. Smith. How much light reaches the retina. Documenta Ophthalmologica Proceedings Series. 1997, 59: 491-511.

[8]C. Curcio. Human photoreceptor topography. J.Comp. Neurol. 1990, 292: 497-523.

[9]F. Shevlin. Phase randomization for spatio-temporal averaging of unwanted interference effects arising from coherence. 2018, 57(22): E6-E10.


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