In order for laser displays to reach a tipping point and become commercially viable there needs to be an answer for the question: “what about laser speckle?” For those unfamiliar, laser speckle is the shimmering, noise-like, variations in brightness we see when we look at the reflection of laser light on a rough surface (which is basically any surface used for projection display). Laser speckle is very distracting for display in the same way white noise is very distracting when listening to the radio, yet there is not a viable way to “tune out” speckle in displays.
Laser speckle is an interesting challenging facing laser displays because there is very little information on how to properly measure it. Being able to measure speckle is of course a prerequisite of determining the best way to reduce the speckle contrast. As the saying goes; “if you can measure it, you can improve it.” So from Alces’ perspective, if we can develop tools, practices, and understanding on how to measure speckle, we can provide resources on how to improve, i.e. lower the speckle in a wide variety of laser projection displays. We’ll be going into more details on our metrology tools in the future but first we wanted to show a basic example of imaging speckle and how the speckle contrast can be reduced by producing independent speckle patterns.
These four images were captured using Alces speckle metrology tools by imaging a monochrome, uniform field of green laser light (the images are in black and white as captured by the camera, and I’ve adjusted the brightness of the images slightly to make them easier to view). There are many important considerations to imaging speckle with a camera, which we’ll discuss later, but they key theme is that the camera should mimic the imaging characteristics of the human eye in order to create a measurement that matches the our own perception of speckle.
N=1With an LED (rather than a laser) this image would appear as uniform bright field but because laser light is coherent (and LEDs are not), the reflection off the rough surface causes the beam to interfere with itself and create speckle. This is a “pure” speckle pattern, equivalent to a single, unique, speckle pattern (N=1) produced by illuminating a polarization-maintaining screen. The speckle contrast, which is measured as the relationship between the standard deviation and the mean, is 100% (we’ll present more details the definition of speckle contrast in upcoming posts). If this speckle contrast was produced by laser display, any video or image would be nearly indistinguishable from the speckle noise. This is of course the worst-case scenario.
N=2While there are numerous ways to generate two “independent” speckle patterns (N=2), the most straightforward is to use a polarization-scattering screen e.g. a white wall. This, in effect, produces two independent speckle patterns which do not interfere with one another but rather add. This is the most basic form of speckle reduction. This additive process is the key to lowering the speckle contrast, and as you can see produces an image with roughly 70% speckle contrast. There is a well-established mathematical foundation which describes this additive process and how speckle contrast relates to the summing of independent speckle patterns, however we’ll save those details for later.
N=4Creating four independent speckle patterns (N=4) reduce the contrast even further; this image shows a contrast of ~50%. Creating many independent speckle patterns is the fundamental challenge facing laser displays because there are only a limited number of variables which can be manipulated. We’ll discuss those variables in the future.
N=100With a 100 unique, independent, speckle patterns (N=100) the ideally-uniform field begins to emerge. Fundamentally, speckle is a noise variable, it can’t be completely eliminated but the speckle contrast can be reduced below human perception. At 100 speckle patterns the speckle contrast is roughly 10% and approaching the human perception threshold.
If you’d like to learn more don’t hesitate to contact us by phone or email, we are always willing to “talk speckle.” And if this information was useful let us know with a comment or feedback. We see speckle as a very interesting technical challenge and are hoping to create valuable resources for those involved with laser displays.