# ND filter for better black levels



## sga2

Anyone try a Neutral Density filter on their projector to improve black levels?


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## thrillcat

Interesting thought. For someone with plenty of brightness on the top end, it is something that could work. Might have to look into that.


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## 73shark

Since contrast seems to be the gold standard for PJ comparison, I suspect if ND filters improved it, they'd be doing it. But if it works, let us know.


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## sga2

I got a set of three (ND2, ND4, ND8) filters from Amazon. They do exactly what I was hoping, reduce the black level. 

With the ND4 filter in place, after recalibration, the black bars above and below a 2.35:1 image are only barely visible on the wall when the main screen is completely back, and not noticeable at all with even minimal image content (starfield, end credits, etc.). The background of space in a starfield image is only a shade brighter than the velvet frame and the difference is only noticeable when looking directly at the border. I need to spend some time tweaking, but am very happy so far.

I recalibrated the projector using Spears and Munsil HD Benckmark. The overall image is darker, but you become adjusted to it easily. Without the filter the image is very, very bright on low lamp. I'm using a Sony VPL-HW40ES on a 115" 2.35:1 Silver Ticket white screen from 15ft. Filter is on an adjustable mount and I use it (and the related calibration settings) only for movies.

I need to borrow a decent camera that I can mount on a tripod and use manual iris, ISO, and shutter settings so I can take some comparison pictures.


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## Alan Brown

This technique has been used for decades. A similar solution is using a gray screen. The two techniques lower black level but do not improve dynamic range, since they also lower white level. 

The gray screen may actually be preferable, due to the added benefit of rejecting ambient room light and reducing reflected light from room components and surfaces. However, as you noted, photographic quality ND filters are available in various increments. That is difficult to duplicate with gray screen options.

Best regards and beautiful pictures,
Alan Brown, President
CinemaQuest, Inc.
A Lion AV Consultants affiliate

"Advancing the art and science of electronic imaging"


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## sga2

Alan Brown said:


> This technique has been used for decades. A similar solution is using a gray screen. The two techniques lower black level but do not improve dynamic range, since they also lower white level.
> 
> The gray screen may actually be preferable, due to the added benefit of rejecting ambient room light and reducing reflected light from room components and surfaces. However, as you noted, photographic quality ND filters are available in various increments. That is difficult to duplicate with gray screen options.


I had a very hard time finding a good grey screen that doesn't have visible texture. Never found one. The ND filter also nearly eliminates the black bars projected above and below my 2.35 screen, which a grey screen can't do.


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## Yiannis1970

ND filters block projector's output light. For instance, a ND2 filter will cut out ~50% of light (last time i measured it was 48%). By reducing the light surely you will get better blacks but at the same time more gray whites. Of course, the use of an ND2 filter always remains a valid option for those light cannon projectors on smaller screens (<100'').


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## Alan Brown

sga2 said:


> I had a very hard time finding a good grey screen that doesn't have visible texture. Never found one. The ND filter also nearly eliminates the black bars projected above and below my 2.35 screen, which a grey screen can't do.


The screens Joe Kane worked with Da-Lite on developing are exceptionally smooth and uniform. The model line designation is "HD Progressive." Gain options include .6 and .9 gain: http://da-lite.com/screen-surfaces/front-projection/high-resolution : http://www.technologyintegrator.net...ou-knew-screen-technology-may-change-412001/1 ; http://www.projectorcentral.com/da-lite_affinity.htm


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## rab-byte

Stewart and Screen Innovations (SI) both have some very good negative gain screens. 

When going budget friendly then Da-Lite is a really good performance to price ratio.


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## Gary Lightfoot

ND filters just reduce the white and black levels equally, so don't change the on/off contrast (cheap resin/plastic ones can adversely affect ANSI contrast though) and don't alter the screens reflectivity so don't help with what the room may be doing to the image. Because we don't see linearly, and because our eyes have an auto white balance and adaptability, sometimes the ND filter can give the impression that it has reduced the black level more than the white level, so on some screens can have a very positive impact. Also, by reducing an overly bright image, they can make the image look more 'cinematic' and less like a bright tv/video. An ND is also removable and can give you back more light as the lamp dims (though it's a coarse adjustment), or if needed for 3D or if you have the guys round to watch some sports with some lights on and a few beers.

Another option is a colour correcting filter (like an fl-day for many UHP lamped pjs) that will reduce the black and white level, and in conjunction with greyscale recalibration can increase on/off contrast. Because of the recal, you tend to gain a few more lumens back (increase white level) so it doesn't reduce the white level so much (and that's where the on/off gain comes from).

Grey screens will reduce the white and black levels in a similar fashion, but to a lesser degree - they're usually around 0.8 gain (some even less, but not 30% less like an ND2), though some with an optical coating can be around, or just over unity. Grey screens reflect less light than a white screen, so less light can reflect back from the walls/ceiling, and this helps to preserve the ANSI contrast in a poor room (it still won't be as high as the pjs native). A little gain helps further by aiming the reflected light more towards the seating area than the walls. The grey base also means that 'black' will look darker than a white screen, and in a poor room with light decor and ambient light will make an image far more watchable whereas an ND will not. A grey screen may have a colour shift which will need calibrating out moreso than a white screen (usually). A grey screen is a bit more permanent and less flexible than an ND, though you could always have two screens, but that's a bit more expensive.

Both have pros and cons and do different things, so you have to decide which route is best for the conditions you have and for what your goals are.

Gary


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