Foot lamberts: how low is too low?


Active Member
After debating for forever on getting a second hand lcd I'm going to travel the different road and intend to get a Sharp 201 dlp but I have a query about the brightness levels.

The pj will be portable and placed where needed and I'll be projecting onto a 7.5ft wide 2.35:1 diy screen (used to be 16:9). Whilst not pitch black the room is reasonably to very dark depending on time of year (only use pj in the evening). The calculator at pj central is based on a 16:9 screen which I assume throws the results out a bit for my purposes. I used it based on an 86" width (which the screen is taking into account 2 x 2" black border) and the throw distance is 10.5ft which is just about right.

Now, the light output at this distance comes out at 9fL. Is this too low? In theory using the zoom I can move the pj forward up to a couple of feet which brings the level up to the magic 12fL. Likewise projecting 1.85:1 movies onto this screen will mean an image width of rougly 75" which again will give me 12fL (though the pj will have to be moved forward a couple of feet).

I've previously borrowed a Hitachi lcd from work that had the same throw distance. According to the calculator, at 10.5ft the Hitachi is hovering around 10 - 11fL and the image looked fine to me; didn't check the bulb hours but they would have been in the hundreds.



Peter Parker

Distinguished Member
If the pj has a reasonable amount of contrast (maybe over 2000:1), then as little as 7ft lamberts in a fully light controlled room is watchable IMHO. I've even found 2000:1 CR at D65 OK with just over 4ft lamberts, so if you can live with cinema levels of brightness rather than tv or plasma levels, you should be OK. CRT owners often have much less than 12fl, but don't complain about a dim image, and I think this may be in part due to the contrast.

You can boost image brightness further by using a screen with gain. 1.3 is usually OK so if you have 9fl you will get 11.7. It will still dim over time as the lamp ages of course so a smaller screen might be the way to go. Try the pj at various sizes and see what looks good to you. Then get a screen to match with some gain (or maybe slightly smaller screen to take into account dimming).

I haven't looked, but projector advertsied figures tend to be a fair bit higher than what you get out of the box, so the numbers may be lower than you think.



Active Member
Thanks Gary - that's helpful. It sounds like I'll be ok - and my screen being 'adjustable' means I can always modify to suit.

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