Light Discolors Paper

Status
Not open for further replies.
J

jmellc

Senior Member
Location
Durham, NC
Occupation
Facility Maintenance Tech. Licensed Electrician
We are upgrading lights in a printshop from T12 HO to T8 HO. Last week, they showed me a press under a light I recently upgraded. Treated paper was loaded in press and sat idle a day or 2. The light supposedly turned the paper from white to beige or a little darker. A press nearby with the same paper is under the old lights and has not had this issue.

Our dealer tells us the T8's are cool white, same as the old T12's. They do not have CW on the label anywhere but he insists they are the same and they look very close.

Has anyone else seen this issue? I have never seen it happen before or heard of it. I have seen sunlight discolor materials but never artificial light.
 
jmellc said:
We are upgrading lights in a printshop from T12 HO to T8 HO. Last week, they showed me a press under a light I recently upgraded. Treated paper was loaded in press and sat idle a day or 2. The light supposedly turned the paper from white to beige or a little darker. A press nearby with the same paper is under the old lights and has not had this issue.

Our dealer tells us the T8's are cool white, same as the old T12's. They do not have CW on the label anywhere but he insists they are the same and they look very close.

Has anyone else seen this issue? I have never seen it happen before or heard of it. I have seen sunlight discolor materials but never artificial light.
Click to expand...

Usually it's the UV that discolors things, but glass will stop something like 95% of UV from passing through, so I can't see the luminaire being the culprit, unless the T8's have a very different envelope. Did they try taking the paper from the press under the new lights and looking at it under the old lights? The local color values can be very different but that might not be obvious with the presses even if side by side without looking at the same sample under each.
 
jmellc said:
We are upgrading lights in a printshop from T12 HO to T8 HO. Last week, they showed me a press under a light I recently upgraded. Treated paper was loaded in press and sat idle a day or 2. The light supposedly turned the paper from white to beige or a little darker. A press nearby with the same paper is under the old lights and has not had this issue.

Our dealer tells us the T8's are cool white, same as the old T12's. They do not have CW on the label anywhere but he insists they are the same and they look very close.

Has anyone else seen this issue? I have never seen it happen before or heard of it. I have seen sunlight discolor materials but never artificial light.
Click to expand...

The easiest way to test this is to take a test sheet and lay it out with two shadow items on it. One shadow item should be raised off the paper an inch or so to allow air flow over the paper and the other should be touching the paper to limit air contact. This will help you figure out whether it's the light or maybe there's a chemical in that area that's reacting with the paper. Either way, I would want to be certain that it's the new tubes.

If it is the new tubes, it's probably a UV or near UV level that's higher than what the treatment in the paper likes. New T8 lamps are usually going to have a better spectrum than the older T12s, so it could be that phosphors were added that are activating a chemical in the paper. It's either below 500 nM or 400 nM that's problematic, and there are sleeves available to combat this.

http://www.uvps.com/product.asp?code=FILTER+++I
 
T8 lamps attain their higher efficiency by using different phosphors, so yes, the color spectrum is different. May not be noticeable to the average human eye, but a friend of mine in the printing industry says it is a problem for them when customers come in for press checks and the colors they see in the press room where they still have high bay T12 HO lamps look different than they do in the office that has new T8 lamps in the grid fixtures. He says he is constantly having to explain this to people who don't know, so it's a bit odd that your printer has never heard of it.

Besides the sleeves, you can buy coated lamps with UV filters too. Maybe the old T12 lamps were coated and you can't tell easily.
 
Press checks should only be done at a color corrected station made for this purpose. These have special lighting to properly judge proofs.

If this is pressroom lighting issue I am sure the Lamp manufacture is aware of issues like this.
 
If its a stack of paper (like you have in a copier), or more likely a large roll of paper, I don't see how the light could affect anything other than the top sheet, or the outside turn. And maybe the very edges of the paper.

But the light isn't going to penetrate into a roll or stack.

If the middle of the roll or middle of the stack is discolored, its not the light.
 
steve66 said:
If its a stack of paper (like you have in a copier), or more likely a large roll of paper, I don't see how the light could affect anything other than the top sheet, or the outside turn. And maybe the very edges of the paper.

But the light isn't going to penetrate into a roll or stack.

If the middle of the roll or middle of the stack is discolored, its not the light.
Click to expand...

The paper discolored on the part exposed to the light up top and on the edges of the roll beneath, where also exposed to light; similar to a closed book with pages discolored at outer edges.
 
Sounds like this paper has very poor resistance to UV. and is actually photo sensitive. How quickly is it taking for the paper be discolored.
 
Sierrasparky said:
Sounds like this paper has very poor resistance to UV. and is actually photo sensitive. How quickly is it taking for the paper be discolored.
Click to expand...

They loaded the paper Friday morning, didn't use the press, then noticed it about Tues morning.
 
gadfly56 said:
Usually it's the UV that discolors things, but glass will stop something like 95% of UV from passing through, so I can't see the luminaire being the culprit, unless the T8's have a very different envelope. Did they try taking the paper from the press under the new lights and looking at it under the old lights? The local color values can be very different but that might not be obvious with the presses even if side by side without looking at the same sample under each.
Click to expand...

It's not just appearance. The paper is literally discolored. I know a lot of colors appear different to they eye in different types of light. I first thought that was what he was telling me, but he rolled the paper forward and showed me where a section under a roller was still white, where not exposed to the light.
 
jmellc said:
It's not just appearance. The paper is literally discolored. I know a lot of colors appear different to they eye in different types of light. I first thought that was what he was telling me, but he rolled the paper forward and showed me where a section under a roller was still white, where not exposed to the light.
Click to expand...

Wow. Things that make you go, "hmmmmm".
 
Status
Not open for further replies.
Top