its all about about distance...

sdahan · 15-10-2009, 8:38 PM

hi,

i wanted to ask the professional help from you...

i needed to calculate the ANSI i will need in a some room, i bought the F22 of projector design with 3000 and i think its not well,

can somone give me a way to calculate the lumance and the contrast i need, considering the light in the room, the distance from the wall, and the rejection of the wall (and maybe some other parameters i didnt consider)

please help me, its urgent,,

thenk you very much!

Gary Lightfoot · 15-10-2009, 10:49 PM

I#m not a professional but I'll help if I can.

If you mean ANSI lumens, then 3000 (if it's accurate) should allow you a pretty large screen if you want the kind of image brightness you'd see at the cinema. How big do you want to go? 20 foot wide 16:9 scren would probably look OK with amount of lumens. Smaller and you may need an ND filter to dim it down a bit (unless you like bright images or need the extra brightness to counter ambient light).

For reflectance, you divide the lumens by the screen size (in square feet) and multiply by the screen gain (result is foot Lamberts)
.

To get the best on/off you need to set you brightness and contrast using a test disk, and make sure there is no ambient light in the room. For best ANSI contrast you need dark walls or a grey screen. You might want to project your image onto a wall first and see what size and room reflections give best results.

Gary

sdahan · 16-10-2009, 9:16 AM

first of all thanks,

i can detail the size of the screen in that case, but i want to pick the right projector in every site i need to plan...

i remember from my physics study in high school, that you can calculate the light on the screen giving the power of the source light and the distance of the screen.

i have a couple of quastions:

1.do you know such a formula? that i can use, giving the distance and the ambient light and the distance, can i calculate how much brigthness do i need and how much contrest?
i want to be able to choose the correct projector for every room, do i need 4000 ANSI or i can mannege with 2000 ANSI?

2.is there a role of how much light there is need to be on the screen, in order to see correctly? - i know that its depends on the one thats looking on the screen, but are there any minimum boundery or maximum boundery?

i really want to belive that our desicions need to be correct by the math and not by our feelings and experience....

thanks again,

AussieTech · 16-10-2009, 10:24 AM

You can use Projector Central's online calculator which will take into effect screen gain, room brightness, image throw and other factors. The only negative thing is you need to pick a projector first.

Projection Calculator Pro - Projector to Screen Distance

sdahan · 16-10-2009, 10:52 AM

thanks but its jst the opposite from what i wanted...

i would like to do this calculation alone....

SteveCritten · 16-10-2009, 5:28 PM

The variables are too vast mate, firstly you have to know the budget. Then, not all PJ's that say they are 3000 lumens are actually that because they vary with bulb age this also applies to the contrast ratio.

I would love it to be as easy as a mathmatical equation. Unless every room is exactly the same and a basement with no ambient light!!

I do believe you can get metres that measure the PJ output when in situ so you can make adjustments for a better picture.

Gary Lightfoot · 16-10-2009, 8:31 PM

For projectors, you usually try to aim for around 12fL in a darkened room (see calculation I gave earlier - result is in fL). Contrast is dictated by the projector and its calibration, and by any ambient light in the room. For light coloured rooms a grey screen usually works better, and you may need more lumens.

If the pj has a lens with constant aperture, the distance of the pj to the screen will have no impact on the reflectance, since the the same lumens are being spread across the same area. However, most lenses vary with the amount of zoom, and you often find that with the pj zoomed smaller, it will have higher contrast but lower lumens (you have to move the pj further back to fit the screen). With image zoomed bigger, the converse will apply.

White walls will reflect light that bounces from the screen back onto the screen, and if the image has dark parts in the image, the reflected light can reduce the visible detail or wash it out somewhat. It can depend on how close the screen is to the walls, but generally you want dark decor for best results.

Do you intend on installing pjs as a profession or as a hobby since it sounds like you have lots to fit?

Gary

sdahan · 17-10-2009, 7:03 PM

i heard that the manufecters lumens is not the real the lumens, i know that the older the lamp is the lumens is reducing.

but when the lemp is new i expect to get the max brightness there is.

somthing that bothers me - contrast is the ability to show different colors, i have a 1:2500 contrast in the projectordesign F22 sx+ and i still has really blur colors, is that depends on the brightness? or only the contrast?

is the contrast chages with the distance? or the screen image? or the embient light?

thanks,

Gary Lightfoot · 17-10-2009, 10:18 PM

Most manufacturers advertised lumens are usually with an image that is close to the lamps native spectrum and not a colour temperature suitable for video. Setting the colours for video usually reduces the blue and green output which in turn reduces lumen output, and contrast. Ideally you need to measure it yourself. Lamps do dim with age but that won't effect the contrast as black and white levels will dim equally. You can take that into account for your fL figure when working out screen size.

If the colour looks blue, then the red green and blue need to be calibrated individually to achieve a white that is close or at a colour temperature referred to as D65.

On/off Contrast capability will change with the lens aperture (zoom) but ANSI contrast (brightest and darkest part of an image) will depend on the brightness of the walls and/or ambient light. Contrast is the range between white and black, and ideally measured when calibrated at D65.

Gary

AussieTech · 18-10-2009, 9:39 AM

Your brightness/contrast and colours could also depend on how you are sending the signal from your source to your projector, and also take into account the capacitance of the cable you are using.

You may need to invest in a DA to adjust the level (brightness/contrast) and also the peaking (sharpness) of the signal.

Gary Lightfoot · 18-10-2009, 10:16 AM

Calibration at the pj usually takes into account source differences and all adjustmenst should be done in the pj if possible. An external scaler with 11 point (or more) gamma/greyscale adjustment can be useful if the pj doesn't have this, or better still, a complete CMS.

Unless you have a bad cable, it shouldn't produce visible or measurable differences. There is an avforums video on HDMI cables that's worth a watch. There is a lot of debate over cables but I won't go there.

Sharpness can add ringing to the image, so use with care. Use a test disk to set the levls and sharpness. See my basic calibration link for more info, and watch the avforums calibration videos (links are there too) which are excellent.

Gary