Back around 2007, when I was at Syndiant, we started looking at the pico projector market, talked to many of the major cell phones and several PC companies, and almost everyone had at least an R&D program working on pico projectors. Additionally, there were market forecasts for the rapid growth of embedded pico projectors in 2009 and beyond. This convinced us to develop a small liquid crystal on silicon (LCOS) microdisplay for embedded pico projectors. With so many companies saying they needed pico projectors, it seemed like a good idea at the time. How could so many people be wrong?

Here we are six years later, and almost no pico projectors are embedded in cell phones or much else. So what happened? Just about the same time we started working on pico projectors, Apple introduced their first iPhone.    The iPhone overnight roughly tripled the size of the display screen of a smartphone such as a Blackberry. Furthermore, Apple introduced ways to control the screen (pinch/zoom, double-clicking to zoom in on a column, etc.) to better use a small display.   Then to make matters much worse, Apple introduced the iPad and the tablet market took off almost instantaneously.    Today we have larger phones, so-called “phablets,” and small tablets filling in just about every size.

Additionally, I have written about before the use model for a cell phone pico projector shooting on a wall doesn’t work.   There is rarely a dark enough place with something that will work well for a screen in a convenient place.

When you carry a pico projector and screen with you, a thin iPad/tablet works better, you can carry it around the room easily, and you don’t have to have a very dark environment. I found that to use a pico projector; I had to carry a screen (at least a white piece of paper mounted on a stiff board in a plastic sleeve to keep it clean and flat) with me.   Then you have the issue of holding the screen up so you can project on it and then find a dark enough place that the image looks good.

The above is the subjective analysis; the rest of this article will give more quantitative numbers.

The fundamental problem with a front projector is that it has to compete with ambient light, whereas flat panels have screens that generally absorb 91% to 96% of the ambient light (thus, they look dark when off).     While display makers market contrast numbers, these very high contrast numbers assume a dark environment; in the real world, what counts is the net contrast, that is, the contrast factoring in ambient light.

Displaymate has an excellent set of articles (including SmartPhone Brightness Shootout, Mobile Brightness Shootout 2, and Smartphone Shootout 2) on the subject of what they call “Contrast Rating for High Ambient Light” (CRHAL), which they define as the display brightness per unit area (in candela’s per meter squared, also known as “nits”) of the display divide by the reflectivity of ambient light in percent by the display.

Displaymate’s CRHAL is not a “contrast ratio” but a good way to compare displays with reasonable ambient light. Also important is that a front projector does not take much ambient light to dominate the contrast. Even dim room light is “high ambient light for a front projector.”

The total light projected out of a projector is given in lumens, so to compare it to a cell phone or tablet, we have to know how big the projected image will be and the type of screen.   We can then compute the reflected light in “nits,” which is calculated by the following formula Candelas/meter² = nits = Gn x (lumens/m²)/PI (where Gn is the gain of the screen and PI = ~3.1416).   If we assume a piece of white paper with a gain of 1 (about right for a piece of good printer paper), then we have to calculate the screen area in meters-square, multiply by the lumens, and divide by PI.

A pico projector projecting a 16:9 (HDTV aspect ratio) on a white sheet of notebook paper (with a gain of, say, 1) results in an 8.8-inch by 5-inch image with an area of 0.028 m² (about the same area as an iPad2 which I will use for comparison).    Plugging a 20-lumen projector into the equation above with a screen of 0.028 m² and a gain of 1.0, we get 227 nits. The problem is that the same screen/paper will reflect (diffusing it) about 100% of the ambient light.   Using Displaymate’s CRHAL, we get 227/100 = 2.27.

Now compare the pico projector numbers to an iPad2 of the same display area, which according to Displaymate, has 410 nits and only reflects 8.7% of the ambient light.   The CRHAL for the iPad2 is 410/8.7  = 47.   What crushes the pico projector by about 20 to 1 with CRHAL metric is that the flat panel display reflects less than 10th of the ambient light, whereas the pico projector’s image has to fight with 100% of the ambient light.

In terms of contrast, to get a barely “readable” B&W image, you need at least 1.5:1 contrast (the “white” needs to be 1.5 brighter than the black) and preferably more than 2:1.   To have moderately good (but not great) colors you need 10:1 contrast.

A well-lit room has about 100 to 500 lux (see Table 1 at the bottom of this article) and a bright “task area” of up to 1500 lux.   If we take 350 lux as a “typical” room, then there are about 10 lumens of ambient light for the sheet of paper screen in our 0.028 m² image used above.   Thus our 20-lumen projector on top of the 10 lumens of ambient has a contrast ratio of 30/10 or about 3 to 1 which means the colors will be pretty washed out, but the black-on-white text will be readable. To get reasonably good (but not great) colors with a contrast ratio of 10:1, we would need about 80 lumens.   By the same measure, the iPad2 in the same lighting would have a contrast ratio of about 40:1 or over 10x the contrast of a 20-lumen pico projector.   And the brighter the lighting environment, the worse the pico projector will compare.    Even if we double or triple the lumens, the pico projector can’t compete.

With the information above, you can plug in whatever numbers you want for brightness and screen size; no matter the reasonable numbers you plug in, you will find that a pico projector can’t compete with a tablet even in moderate lighting conditions.

After working on the problem for several years, it became clear that rather than adding a pico projector with its added battery, they would be better off just making the display bigger (ala the Galaxy S3 and S4 or even the Note).   The microdisplay devices created would have to look for other markets, such as the closed eye (for example, Google Glass) and automotive Heads Up Display (HUD). And all this is before considering the power consumption and space a pico projector would take.

Table 1. Typical Ambient Lighting Levels (from Displaymate)