# Custom Made Exposure Unit Light Distance Question



## hdem (Mar 1, 2008)

Hey guys,

I read on other threads that 1.5 times the diagonal of the screen for the distance of the light is the formula you want to follow but this causes some physical problems...

I am going to use a 1000w metal halide lamp with a reflector box behind it that measures 19 x 16 inch. I want to build the unit to accommodate the rare times where I have to expose an "all over" screen size of 42x48 OD but I usually use 23x31 OD.

Setting the glass 1.5 times the diagonal even for the smaller screens would still be 5ft off the ground and 8ft for the over sized screens! This can't be the case so I was wondering if I am using a powerful 1000w metal halide bulb instead of a 400w or 500w I read others used, can I have the glass closer to the light so that it's maybe only 30 inches away? Or is it just simple physics that no matter how bright the bulb it still needs to be 1.5 the diagonal? Remember that I do have a reflector behind the bulb...


Here is a link to the balast/bulb system I bought:

Grow Light Systems > 1000 Watt Metal Halide Grow Lights

it has lots of information about the system on there


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## TerryCombs (Nov 11, 2009)

For a pinpoint light source (bulb and not tubes) your distance should be *equal* to the diagonal distance. Cramped for space, measure the diagonal of your largest artwork and you should still be OK. 

You can also set up with your light source free-standing and your glass standing up rather than horizontal. Big shops burning multiple screens at once have vacuum tables that are on a swivel to stand up the glass before exposure. Then, you can change your distance as needed.

Hope this helps!


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## tman07 (Nov 14, 2007)

I have a 1000W metal halide and it is 30 inches from glass.

If found that if it was any closer, I was getting underexposed out on the edges of the screen
and over exposed in the center.

my screens are 23 x 21 inches.

I use premixed emulsion and my exposure is 35 seconds.


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## RichardGreaves (Nov 7, 2006)

*Exposure Unit Light Distance*

Most of us are stuck with the equipment we can afford, yet I know what I will buy when I win the lottery.

If your lamp distance is the diagonal of your largest artwork - the light rays will strike your positive _at the outside edges_ at a 63° angle compared to 90° in the center. Not the best, but what can you do? This is why some shops swear by their fluorescent exposure units - especially large screens where they need even exposure and don't print 65 line halftones. 

There is usually a small (?) 3-4 inch difference in the actual distance those outside light rays have to travel in the t-shirt market. 

*Dull part*
There's a principle in physics called the Inverse Square law. Sound and electromagnetic radiation like UV and IR energy all follow this law.

The intensity of light falling on a flat surface from a point source is inversely proportional to the square of the distance. Dizzy?

The lamp doesn't change it's intensity, so the same amount of light has to be spread over a larger area if it is further away - Like the same amount of food you cooked has to be distributed to all those new uninvited guests at your dinner party.








Using the Inverse Square law, UV energy falling on the outside edges of your positive at the "diagonal distance" gets about 80% of the energy the center gets. This is where a stencil's exposure latitude (tolerance), helps minimize that difference. 

_Less than the image diagonal_ with with a speedy SBQ stencil and you can underexpose the outside, yet over expose the center.

At 1.5 "diagonal distance", the outside edges of of your positive gets about 90% of the energy the center gets, at an angle of 71.5° compared to 90° in the center.

Yes, the exposure must be longer because 1.5 is further overall, but the difference between the outside edge and the center are less.

You will always be OK, because you're stuck with the equipment you have. Each variable effects the final outcome - which you can measure and calibrate your artwork & positives if you're off target.


If you want to study the math, call me or SKYPE me.


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## hdem (Mar 1, 2008)

I sort of understand all that info Richard but what does the reflector behind the light do to the energy? Will it focus the beam more or will it help it spread? The reflector has been designed to minimize "hot spots", do you think this helps for exposing screens? Also does the reflector make things worse because it will send the light at different angles rather then straight forward light your diagram shows?

I left a link to the lamp I bought in my original post which says a lot about it.

Thank you!


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## RichardGreaves (Nov 7, 2006)

*1000w Grow Light reflector*

Are you having buyers remorse?

Most of us are stuck with the equipment we can afford. As time goes on you will judge the variables of your new lamp in preparation for you next lamp purchase.

Electrical power variations
The lamp type
Reflector
The lamp power - Electrical watts in, then UV-A output
The age of the lamp
The UV-A energy output range
The lamp distance to stencil
Vacuum frame


A reflector re-directs light emitted at the back of the lamp toward the plant or stencil. Access Discounts says it reflects 95%.

The reflector won't change the angle of light striking the outside edges of your image or the power of the light - unless you remove the reflector.

Plants don't grow better with parallel rays to minimize under-cutting. Fewer 'hot-spots' sounds good.

Many DIY screen maker use US$30 quartz halogen work lamps and make money screen printing - even with the horrid reflectors they have.








_US$30 Quartz Halogen reflector light pattern_

You're bought an experimental, "bang for your buck" bargain, not a graphic arts exposure lamp. I'll bet - for you - a commercial lamp isn't worth the money for what you get. Why buy a Porsche to go to the grocery store? I live in Brooklyn and a US$20 wire cart works better than a Porsche.

Like all first purchases, *I'll bet you love it*, until you want to expose faster or sharper. I loved my Volkswagens in high school, but in the back of my mind I knew there were better cars.

Sharper fine lines and halftones rely on the vacuum as much as the lamp. Vacuums are hard to DIY.

Exposure penetration relies more on the amount of the targeted UV-A energy. Speed cost money. How fast do you want to go?

Did you buy a Stouffer 21 Step Gray Scale yet to monitor your exposure?


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## inkforshirts (Mar 22, 2010)

i have 6 unfiltered uv blacklights 6 inches under glass.


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