304 North Cardinal St.
Dorchester Center, MA 02124
304 North Cardinal St.
Dorchester Center, MA 02124
Back around 2007 when I was at Syndiant we started looking at the pico projector market, we talked to many of the major cell phone as well as a number of PC companies and almost everyone had at least an R&D program working on pico projectors. Additionally there were market forecasts for rapid growth of embedded pico projectors in 2009 and beyond. This convinced us to develop 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 6 years later and there are almost no pico projectors embedded in cell phones or much else for that matter. So what happened? Well, 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 make better use of what was still a pretty small display. Then to make matter much worse, Apple introduce the iPad and tablet market took off almost instantaneously. Today we have larger phones, so called “phablets,” and small tablets filling in just about every size in between.
Additionally I have written about before, the use model for a cell phone pico projector shooting on a wall doesn’t work. There is very rarely if ever a dark enough place with something that will work well for a screen in a place that is convenient.
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 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. By the time you carry a pico projector and screen with you, a thin iPad/tablet works better, you can carry it around the room with ease, and you don’t have to have very dark environment.
The above is the subjective analysis, and the rest of this article will give some 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 absorb generally 91% to 96% of the ambient light (thus they look dark when off). While display makers market contrast number, these very high contrast numbers assume a totally 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 it gives a good way to compare displays when in reasonable ambient light. Also important, is that for a front projector it does not take much ambient light to end up dominating the contrast. For a front projector even dim room light is “high ambient light.”
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 all we have to do is 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 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 in to the equation above with a screen of 0.028 m2 and a gain of 1.0 we get 227 nits. The problem is that same screen/paper will reflected (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 really 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 where the pico projector’s image has to fight with 100% 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” up to 1500 lux. If we take 350 lux as a “typical” room then for the sheet of paper screen there are about 10 lumens of ambient light in our 0.028 m2 image from 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 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 and no matter was reasonable numbers you plug in, you will find that a pico projector can’t compete with a tablet even in moderate lighting conditions.
And all this is before considering the power consumption and space a pico projector would take. After working on the problem for a number of years it became clear that rather than adding a pico projector with its added battery, they would be better off to just make 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 near eye (for example, Google Glass) and automotive Heads Up Display (HUD).
Table 1. Typical Ambient Lighting Levels (from Displaymate)
0 lux –
100 lux –
500 lux –
1,000 lux –
3,000 lux –
10,000 lux –
20,000 lux –
50,000 lux –
100,000 lux –
|Pitch black to dim interior lightingResidential indoor lighting
Bright indoor lighting: kitchens, offices, stores
Outdoor lighting in shade or an overcast sky
Shadow cast by a person in direct sunlight
Full daylight not in direct sunlight
Indoor sunlight falling on a desk near a window
Indoor direct sunlight through a window
Outdoor direct sunlight