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Lens Flare
The hidden enemy of RPTV black level performance

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Tip by Dave Gibbons: 

Introduction

While one expects CRT-based rear projection televisions to deliver good black levels, there is a factor that compromises black level performance in these displays.

This factor is something called lens flare. Lens flare is most commonly seen in photographs. It usually occurs when the photographer is shooting in the general direction of the sun. Bright sunlight enters the lens at an angle and bounces around inside the lens body. As it reflects off of the interior parts of the lens, some of it ends up getting on to the film, usually in the form of bright circles or shapes, smears of light, or lines.

Binocular, telescope, and camera lens manufacturers all try to make the inside of their optical assemblies as dark as possible and also add light baffles to try to waylay the misdirected light so that it does not spoil the image

Rear projection televisions also suffer from lens flare. The best place to see lens flare is usually during the end titles of a film. Often the titles are bright white on a black background. Look for a small single title, and pause the player at that point. Look at the black areas of the screen around the title. Is there a halo, ring , or general smear of light of the same color as the title? That unwanted light is most likely due to lens flare.

If you want to see the flare at its source, pull the screen off the TV and look into the lenses while a bright field is being displayed by the TV. Look for light being reflected off of shiny interior parts of the lenses by moving your head around so you can look down into the lenses and see the insides of the lenses. (Don't run your head into the mirror while trying to do this.) The light bouncing off of those interior edges and surfaces is the problem.

Your " black screen " black level very probably is quite good on your RPTV, especially if you have done the work to line the interior of the cabinet with light absorbing material, and view your TV in a darkened room. However, one small bright object on a otherwise black screen may cause light pollution on the supposedly black areas of the screen due to this problem. In images with a lot of bright areas mixed with some supposedly black areas, the black level of the supposedly black areas will suffer considerably.

INSTRUCTIONS ON HOW TO REDUCE LENS FLARE

Preparations

All of the above been said, those who are not scared off can remove the first lens assembly. I'm going to assume you have purchased the service manual for your TV so the you know how to do this. (BIG HINT)

Find a CLEAN, WELL-LIT, UNDISTURBED place to work. No kids, pets, significant others, etc. should be able to disturb you.

Get some clean, washed cotton cloths on hand. Used White T-shirts are good choice. Clean white cotton gloves are also a good thing to have when doing this job. In addition, I highly recommend you obtain a source of clean, dry compressed air.

Examine the lens assembly. Determine how the lens assembly comes apart. In the case of the " Delta Digital 265 " lens assembly from Corning Precision Lens Inc, (This assembly is used in the Toshiba 50HDX82 RPTV, and this lens assembly is what I will refer to throughout this discussion.) the threaded screw which carries the wing nut used to lock the mechanical adjustments of focus must be removed. The end of the screw has been squared off. A small "Vise-Grip" plier can be used to gently turn this screw counterclockwise to remove it from the plastic assembly it mounts in.

Then the whole internal lens assembly can be rotated fully clockwise. Note that as you rotate this assembly that two plastic studs are turning in a couple of spiral slots to move the lens assembly up and down in the lens frame. If the lens assembly is rotated fully counterclockwise the studs will hit the end of the slot. You'll then notice that there is a groove running up the inside of the lens frame to allow those two studs to slip up inside the frame so that the inner lens assembly can be removed from or inserted into the frame. You may want to use a small hobby knife to put a bevel on the start of that groove to make it easier to push the lens assembly up out of the frame. Some gentle prying and cursing may also be necessary.

Realize that any contact with the front or rear lens while you are doing this may mean either a dirty or damaged lens. Wearing clean cotton gloves at this point is a good idea.

The lens assembly can now be disassembled by removing the screws holding the two halves together. Remove one half in such a way as to leave the lenses resting in the other half of the lens shell. Make a drawing at this point of exactly how the lenses fit into the lens shell. Which lens goes in which position, and which way does the lens face? Be sure about this, as you don't want to keep handling these lenses unnecessarily.

Note that the Corning lens assembly has glass AND plastic lenses! Glass or plastic, all of them must be handled with the utmost care. ONLY TOUCH THE EDGES OF THE LENSES, AND THEN WITH GLOVED FINGERS. The plastic lenses are shaped like cups, and thus can be safely rested on a flat clean surface with the curved side up. The center glass lens should be supported only by its edges. Some sort of cloth-lined trough of an appropriate size to support the lens should be arranged.

Now that the lenses are out of the way, you can take a look at the plastic shell that held them. In the case of the Corning assembly, the shell is molded out of black, but shiny, plastic. We want to get rid of the shine. A high-quality flat black paint should be applied to all of the interior surfaces of the shell. (A search of the Web recently did not produce any " super " flat black paints available to us ordinary mortals. 3M Corp. used to make something nice, but they discontinued it. Arg!)

I ended up using Badger brand " Model Flex " No. 16-119 flat black paint. This is a water-based acrylic paint of good quality. It is available at better hobby shops.

Brush or spray a THIN, EVEN coat of the paint onto all of the interior surfaces of the lens shells halves. Set them aside to dry thoroughly.

Working on the Lenses

The plastic lenses in the Corning lens assembly do not have painted edges. This allows light to bounce around the inside edges of the lens and bounce back out where it should not. For painting the lenses, a top-quality small brush should be obtained at the same place where you bought the paint. The flat faces along the outer circumference of the plastic lenses, the flat outer edges of the lenses, and the outer portion of the rim of the curved face of the lenses should be painted black. As you might guess, one slip or drip could mean the purchase of a new lens assembly.

Before you start painting, put the lenses back in the shell once the paint is dry in the shell. Look through the lens assembly, particularly at the curved outer faces of the lenses. You want to identify how far in from the edge of the lens you can paint without blocking light coming through the lens assembly. I ended up painting the outer rim of the curved face of the lens on a line about 1/8th of an inch in from the circumference defined by the molded plastic retaining rings on the shell.

If you are uncertain about handling this lens painting part of the job, either skip it , or practice painting objects of the same general shape as the lenses until you feel confident. Do not load the brush heavily with paint, as this will promote paint drips running where you don't want them. This lens painting job must be done very carefully, so do not attempt it when you are rushed or distracted.

The center glass lens in the Corning assembly already had the edges of the lens treated, both with some paint, and with grinding. You could try painting that outer edge black, but I considered that the existing treatment was adequate.

Once the paint on everything is thoroughly dry, it is time to bring out the compressed air. Blow off all dust particles and dirt from the lenses and the shell assembly halves. Use appropriate lens cleaning material and liquid to remove any fingerprints or smudges. This is another place where it is easy to scratch the lenses, particularly the plastic ones. Take your time, and think about what you're doing.

Place the lenses back in the proper order in one of the shell halves. Use the compressed air again to blow the lenses clean one more time and reassemble the shell around the lenses. Inspect the shell and lens assembly for freedom from dirt, and correct any remaining problems.

Reassemble the shell assembly back into the frame, and reinstall the screw that the focus-locking wing nut rides on. Reinstall the remaining hardware on the lens assembly. Re-inspect the entire lens assembly for cleanliness and any other problems. Correct as necessary and then set the lens assembly aside in a clean place.

In the Toshiba 50HDX82, the lens assembly sits atop the main CRT gun assembly. The top of the main CRT assembly just under the lens assembly consists of a cooling liquid-filled chamber with a lens molded into the top. This lens is shaped like a cup, and thus automatically gathers dust particles and dirt at the bottom, right in the main path of light going up to the lens. Use your compressed air to blow that dust out of this cup-shaped lens. The top inside rim of the cup has been blackened, but it still has a somewhat shiny finish which contributes to the lens flare.

A tremendously brave person could try to paint that blackened edge with the flat black paint we used previously on the lenses. I'm not that brave. Instead, here's what I recommend you do:

Order some black "flock paper" from Edmund Optical Supply. This paper has one side which has a light-absorbing texture. Get the thin stuff without the adhesive backing. Cut a ring-shaped piece of this flock paper. (A drawing compass is very helpful in drawing circles of the right diameter on the back of the flock paper to help as a guide for the cutting.) The outer diameter of the ring should just fit into the circular depression which surrounds the cup-shaped lens. The inner diameter of the ring should be small enough to block reflections from the edges of this "cup lens" and other off-axis light without reducing the main light beam brightness too much. I found a 5 millimeter center opening to be about right.

Use small pieces of double-sided tape attached to the back (non-flocked side) of the flock paper ring to attach it in place around the edge of the lens cup. You may want to make several trial pieces with different inside diameters for this flock paper ring before you figure out the best balance between knocking down lens flare at this point in the optical path, and reduction of screen brightness. A smaller center opening in the ring will improve the flare problem, but the main light beam can get choked off too. Another way to estimate this ring's effect is to put the lens back on over the flock paper ring, and look back down through the lens. If the flock paper ring is not visible through the lens once the lens is in place, go smaller on the center hole size of the ring.

Once you are satisfied with your treatment of the area underneath the main lens assembly, clean it out one more time with compressed air and reinstall the lens assembly.

Now you only have two more lenses to do, unless you have a TV with a single lens, which may be the case if your rear-projection TV is LCD, DLP, or some other new alphabet soup technology instead of the old-fashioned CRT's.

Lens assemblies from other manufacturers will certainly be different in detail, but the general sequence outlined above will still apply.

Conclusion and Results

It must be noted that the television on which I did this lens treatment still exhibits lens flare, it just has been reduced due to this effort. Your mileage will vary. Please look at the image that accompanies this article on my web site (www.sonic.net/~dgibbons) to see the difference between a treated and untreated lens assembly.

More expensive rear-projection televisions may use lens assemblies which are less prone to this problem. If I was to buy another RPTV, I would look for televisions with the minimum possible lens flare.

Again, the easiest place to see lens flare is when a single bright object is displayed on an otherwise black screen. If the black areas become lighter when the bright object is present, lens flare should be suspected.

 

David Gibbons
Home Theater Tune-Up
www.sonic.net/~dgibbons

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