Mobile Phone Projectors "Will Launch This Year"

An anonymous reader writes "Mobile phones with built-in mini projectors will launch later this year, according to 3M, which gave PC Pro a hands-on demonstration of the technology at CES 2008. The projector has a brightness of around 8-10 lumens, and is capable of displaying an image of up to 50 in., although 3M's spokesperson Greg Roberts told us that, with perfect lighting conditions, it's possible to squeeze a 60-in. screen out of the projector."

Comments

a basic tutorial

[adam]

Being in the film industry, I work in footcandles usually and not lumens, but If I recall correctly the correlation between them is close (I'm not a cinematographer, so lighting is secondary to my normal job function). A 'birthday cake candle' in a pitch black room, will produce 1 foot candle of light at-- wait for it.. 1 foot. If you put a 1-foot-square surface (like a 12" x 12" piece of paper) 1 foot away from that candle, it will be hit with "1 lumen"

A 'normal' candle produces about 15 lumens. Incandescent bulbs (normal lightbulbs) produce about 15-18 lumens *PER WATT*. So this projector is roughly equivalent to
Now, there are claims of a 50" projection (diagonal, I assume) from this-- no specification as to how far from the projection source the 'screen' is, but light works on the inverse square law-- basically, as you double the distance from a given light source, you get a square root of intensity. So if this sucker threw 10FC at 1 foot, at 2 feet that intensity has dropped to 3.2. At 4 feet, 1.8. So if that 50" screen requires you to be 8 feet back.. forget about it.

Overall, this sounds like a cool little geek gadget, but as other posters have said, probably just another example of cellphones trying to do too much (too poorly ;). If they can increase the light output significantly, it might be useful for something other than showing someone really low brightness web pages shined onto a piece of whitepaper 10" away while in a darkened room.

Apologies if I screwed up any of my tutorial, as I said, I don't paint directly with light, I just admire the guys who do.

That's more like 5 inches

[Aaron Isotton]

What counts in a projector is contrast (e.g. how much brighter is a "white" projected spot as opposed to a "black" projected spot).

Obviously this depends on ambient light, since the darkest part of the screen (i.e. the "black spot") is illuminated only by ambient light (assuming that 'black' in the projector means 'no light passes').

Illuminance is measured in Lux (lx). Lux is defined as follows.

Lux = Lumen / m^2.

Now, a "good" contrast is 10-15, i.e. a white spot will be illuminated with 10-15 times the lx a black spot is.

Normal ambient light is highly variable; a typical table in a lecture room should be illuminated with about 500-1000 lx; the ambient light on your typical screen in an illuminated room (i.e. not a theatre) will be illuminated with maybe 100-500 lx.

So in order to obtain a proper picture a projector should be able to do at least 1000 lx. Comparison: a typical home cinema beamer has about 2000 lumen and projects an area of about 2x1.12m; this means 2000 lumen / 2.24 m^2 = ~900 lx. And guess what, the picture is just fine when the room is "quite dark" and pretty washed out when it is illuminated.

With the claimed 8-10 lumen - let's assume 10 - you can thus illuminate

10 lumen / 1000 lx = 0.01 m^2

Assuming a picture format of 16:9, that's a picture size of

sqrt(0.01 m^2 / (16 * 9)) * 16 = 0.13 m width
sqrt(0.01 m^2 / (16 * 9)) * 9 = 0.075 m height

An incredible 13 cm x 7.5 cm! (5" x 3" for Americans).

That's a diagonal of 5.8". Makes sense since a 2000 lumen projector is 200 times more powerful and accordingly projects an image with sqrt(200) = ~14 times the diagonal.

Except in the darkest of situations, you will *never* have an usable 50 inch image with a lousy 10 lumen.