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Many old television screens have a very prominent "light grey" color to the glass, I'm guessing from the phosphor, create darker areas if the effect of electrons make the phosphor glow?

Is it an illusion due to contrast between the brightly lit and darker unlit phosphor or can the screen actually darken somehow?

Here are some photos of old TVs from the 70s:

enter image description here

Zhro
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7 Answers7

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Forget old fashioned TVs, have you ever watched a PowerPoint in a normally lit conference room? The projector screen is white until you turn on the projector, at which point it seems to become dark grey.

This is because your eyes are sensitive to relative light levels and not absolute. Black doesn't require no light, it's just less light. You can can make a white object dark just by putting something brighter next to it. Projectors, LCDs and CRTs of all kinds exploit this effect.

user1850479
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    My old TV with much light bleeding approves. Constrast is the most important factor. – Sacha Feb 08 '21 at 22:05
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    You can always blow somebody’s mind by pointing out that projectors can’t display black. The darkest black they can show is the white of the wall. – Michael Feb 09 '21 at 09:28
  • Not only does black not *require* absolute absence of light -- it would be atypical. Such absolute absence is not easy to achieve in spaces not designed for it. Even into rooms with no exterior wall surface light seeps in through cracks in doors, panels and ducts, and many everyday devices these days have LEDs. For professional photographic darkrooms, for example, which require extraordinary low light levels, special sealing doors (and blinds, if they had any windows) are required. – Peter - Reinstate Monica Feb 09 '21 at 11:11
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    As an aside it may be interesting that this fact -- that true "black" is almost never encountered -- was realized by the impressionists at the and of the 19th century and influenced the way they painted (see e.g. https://designforhackers.com/blog/impressionist-color-theory/). – Peter - Reinstate Monica Feb 09 '21 at 11:21
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    This unjustly less upvoted answer https://electronics.stackexchange.com/a/547023/17817 has one important point missing in other answers: Old TVs were normally placed in a dark part of the room. A strong light shining on the screen greatly interfered with the picture. – pabouk - Ukraine stay strong Feb 09 '21 at 13:50
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It's basically an optical illusion. I never ever remember anything but a very dark screen when I was a kid waiting for "the box" to warm up and then my mum shouting that the TV needed some money in the slot to make it work (damn those rental TVs in the 1960s).

Consider this picture (taken from this wiki site): -

enter image description here

It looks like the "A" square is darker than the "B" square doesn't it?

But if I use paint.exe to sample A's colour and draw a line from A to B we see this: -

enter image description here

You don't have to believe me. Just copy the top image and try it yourself.

Your brain can play some amazing tricks in trying to make sense of the optical info it receives.

Andy aka
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    Rental TVs? Where it was? – fraxinus Feb 08 '21 at 10:05
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    @fraxinus England in the 1960s – Andy aka Feb 08 '21 at 10:05
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    Thank you. Never even imagined a business model like this could exist back then. – fraxinus Feb 08 '21 at 10:28
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    I rented a washing machine in the UK in the late 90s. You could still rent TVs then. – D Duck Feb 08 '21 at 11:28
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    Rental TVs were quite common. There was a chain called ["Radio Rentals"](https://en.wikipedia.org/wiki/Radio_Rentals) for example. Coin-operated TVs were nowhere near as common, at least by the 80s. Now, instead, there are various forms of [rent-to-own](https://en.wikipedia.org/wiki/Rent-to-own), often with very steep interest charges – Chris H Feb 08 '21 at 13:26
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    The *square* A is darker than the square B, in the sense of being made of a material with lower reflectivity. What you are sampling is not the squares, which exist only in the fictional world depicted by the image, but the rhombuses which exist in the image. The rhombuses are the same colour even though they depict squares which are different. – bdsl Feb 08 '21 at 14:29
  • Between coins for the TV, coins for the heat, and coins for the Magic Fingers, you filled your pockets in the afternoon and they'd be empty by bedtime ... – davidbak Feb 09 '21 at 02:29
  • @fraxinus It was a very good business model from the customer's point of view. CRT-based TVs were not very reliable. A rental agreement was pretty much the same as an insurance policy - if the TV failed you got a quick and "free" replacement, without the hassle of dealing with a repair shop or the unexpected cost of buying a replacement. – alephzero Feb 09 '21 at 04:35
  • If I look off to the side of the image, my brain seems to stop doing the automatic color correction, and A and B look like the same color in my peripheral vision. – user2357112 Feb 09 '21 at 05:04
  • @bdsl talking about that optical-illusion, no, the squares (or rhombus, yes) A and B are in fact identical in color, down to the RGB values. There is no difference, in reflectivity or otherwise - that's the point of the illusion. If you don't believe it - save the image out and examine it in a paint-program. It's fairly mind-blowing, actually - moving square A down to where B is, and *watching* it change "color". – John C Feb 09 '21 at 16:33
  • This is a great example but barely scratches the scope of how our perception of colors and contrast are largely a product of our minds. You can create the perception of colors that are not there. For example, I used to have a room painted green and the hallway outside was yellow. At the right time of day, if you sat the green room, the hallway was clearly solid pink until you walked into it. You can setup scenarios by casting shadows with colored lights that produce this false color effect. – JimmyJames Feb 09 '21 at 19:52
  • Remember this is an EE site @JimmyJames – Andy aka Feb 09 '21 at 20:01
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    @Andyaka Sorry, let me let you get back to discussing rental TVs, then. – JimmyJames Feb 09 '21 at 20:06
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    @JohnC I do believe that the rhombuses are the same colour. I'm making a distinction between the rhombuses, which as you say can be examined in a paint program, and the squares, which can not because they are fictional. They are not the same colour, one just happens to be in a shadow which exactly cancels out the colour difference. – bdsl Feb 09 '21 at 22:17
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It was not watched in as bright ambient light as a good photo of the tv receiver needs when the camera was supported manually. Or the photo was taken with long exposure time having the camera on a tripod. The old CRT television was placed to as dark corner as possible for good contrast. I have had a CRT B&W television and it was useless if the daylight in the room wasn't dimmed by drawing the curtains in front of the window.

Other aspects: The advertisement photos from 1970's can be heavily retouched for the appearance that was then thought to be the most attractive. They can even have "for ads only" dummy CRT or an inserted mask for easy good looking no-reflection photographing. I guess too dark screen could be seen as "it will stay dark".

I have just now one CRT from 1980's in front of me. I brought it out from the storage only to see it due this case. It doesn't look grey, it's a little green. It's B&W phosphor glows white, not green and it looks substantially darker than CRTs in questioner's ads.

  • Another aspect to consider is that given that people would watched televisions in darkened rooms, but television sets were not supposed to draw attention to themselves when rooms were lit. A television set whose screen appeared solid black when it was turned off would have been seen as an eyesore. – supercat Feb 09 '21 at 16:12
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Our eyes are amazingly adaptable to ambient light level:

  • We can see in moonlight (but our colour sensitivity is reduced).
  • We can see in bright sunlight.

Yet in each cases, we can see gradations of light (from dark to light). Even small variations in light level are discernible. In the case of a TV screen that starts out "grey" in ambient light, we can discern an even slightly brighter image. The ambient "grey" background is established in our brain as the darkest part of the image. The screen background does not darken.

You ask if the apparent darkening of the phosphor screen is an illusion. Yes....there are a great many aspects of human vision that are illusory, suggesting that much brain processing goes on between eye and conscious perception.

Advances were made over the years in reducing the reflected ambient room light, yielding a darker background. The photo shows two colour CRT screens.
2003(top) vs. 1980(bottom)
The lower (larger) screen is a 1980's era TV while the upper (smaller) screen is a DELL monitor made in 2003. The DELL monitor certainly reflects less ambient light. It also employs an anti-reflection coating that reduces glare.

glen_geek
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    I believe the main difference between the two screens is that the top one has a dark tint added to the glass covering the screen.  That reduces the _relative_ brightness of reflected light, since it has to pass through the glass twice (once in each direction), while the CRT's light only passes through it once.  Of course, the CRT's brightness needs to be greater to compensate, but by the time this became common (the '90s, IIRC), CRTs were plenty powerful enough. – gidds Feb 08 '21 at 09:51
  • @gidds Tints don't stop the front surface reflection, and that one is the strongest. More likely the newer CRT has an anti-reflection coating. – J... Feb 08 '21 at 16:46
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    @J... The front surface is shiny and mostly gives a specular (directional) reflection; but it's the diffuse reflection off the back surface (and its phosphor coating) that lightens the whole screen.  And the latter is what tinted glass reduces. – gidds Feb 08 '21 at 16:57
  • @gidds Ah, good point. – J... Feb 08 '21 at 17:00
  • @gidds Another CRT surface treatment by Zenith never really caught on - it gave a *really dark* surface reflectivity. https://patents.google.com/patent/US3935036A/en – glen_geek Feb 08 '21 at 17:48
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If you go a bit farther back in time, to so-called "black and white" TV sets, not only are the other answers correct as concerns contrast, but there's a "color correction" that takes place in our neuroptical perception. We all perceived the images as grayscale, of course. However, if you were observing just the overall light produced, perhaps looking thru a window from outside, or thru a doorway from down the hall, the TV output has a light-violet tinge.

Carl Witthoft
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At one of the companies I worked at, there was a room we called "the cave". It was a very dimly lit room for the CAD people doing work with CRT monitors, with the brightness turned down, which resulted sharper images. I still have two CRT monitors (Hitachi CM722, Viewsonic G225FB), and in a dark room, you're getting close to true blacks with these and better CRC monitors. I also have an LCD (IPS) monitor, and in a dark room you see the back lit light coming through producing a dark shade of grey, and brighter if viewing angle is not straight ahead (so if looking at center of screen, the corners are brighter).

However, in a nearly black room, the phosphors on a CRT monitor can give off a glow for a while after the CRT monitor is turned off, and those phosphors can also pick up some energy from ambient light in the room (so just after turning off a light, you get a bit of glow). I assume that OLED monitors or TV's do not have this effect.

rcgldr
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Anything to enhance brightest and contrast also raised the power levels needed to transmit thru the aluminum oxide but also resulted in reflections of the phosphor forward.

They applied a thin aluminum oxide film that appears grey to reflect the light forward and also spread heat evenly to the glass then lacquer coated this.

The glass thickness affect transmissibility and reflections, so later this was also carefully tinted with additives that were also slightly magnetic for other reasons and lead to the glass to block harmful XRays.

Tony Stewart EE75
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