light source (distance) for two screens at a time? 23x31's

[Dottonedan]

I need that old "rule of thumb" info where you x your screen frame size by X to get your light source (distance) for two screens at a time? 23x31's  This is for a wall mount exp.  I think I recall 40" but that can change based on the screen frame size. Here, we are doing two 23x31's placed horizontal or side by side.

Anyone remember what that is?

[Frog]

the rule of thumb without special reflectors is 1.5 times the diagonal, so I figure the diagonal of those two screens next to each other is about 60", so the distance would be 90"

[mimosatexas]

I am about to start doing this due to picking up a huge vacuum frame, and logically for me it would be 1.5x the diagonal of the combined art on both screens.  I would also think it would be good to have the screens rotated 180 degrees so the light source is mimicking hitting both screens from the same angle/side.  See mockup.  distance would be 1.5x the red line, centered.

[ABuffington]

Take the diagonal distance from the corners of both frames side by side.  42-48" is common.  The key is the light needs to hit the glass at less than a 45 degree angle to go through the glass or a large percentage of light will be reflected away.  The other issue is lamp strength.  At 84" the light would be 1/4 the strength of a 42" distance, not half due to the inverse square law of light.  So the best exposures will be in the 42-48" range on metal halide lamps with a short an exposure time as possilbe.  23x31=42" on the diagonal, for halftones backing it up a little to 48 seems to help resolve fine tonals a little better due to light rays hitting a little squarer to the glass. I avoid the 1.5 multiplier mainly due to many shops using wb, discharge and hsa where the emulsion needs to have all the exposure light strength possible to make a durable screen. Works great as well, but you need to dial in different exposure times for 42-48 vs 84-90.  Could be as much as 3-4 times the exposure to equal the strength. Light too close to the screen and you will overexpose the center area where the frames meet, and underexpose the outer edges where the light reflects off due to the angle to the lamp being greater than 45 degrees.

[Frog]

I am about to start doing this due to picking up a huge vacuum frame, and logically for me it would be 1.5x the diagonal of the combined art on both screens.  I would also think it would be good to have the screens rotated 180 degrees so the light source is mimicking hitting both screens from the same angle/side.  See mockup.  distance would be 1.5x the red line, centered.

The drawback of your measuring method is an underexposed screen surrounding the image

[mimosatexas]

The drawback of your measuring method is an underexposed screen surrounding the image

I'm not sure I understand.  If I take my standard art size of around 13x19, centered on a 23x31 screen, the math adds up to a 40" diagonal if the screens are touching. (13+5)*2 as the bottom "side" of the triangle, so 36^2 + 19^2 = c^2, where c = ~40, then x1.5 = 60 inches away.  That should result in fairly even exposure.

Al's math is a little off as well.  The diagonal on 2 23x31's next to eachother (46x31 together) is 55"

[Sbrem]

though we didn't get too overly scientific, we're around 45" with a 5K Metal Halide (Violux 5000S). Our wall unit is 63 x 74...

Steve

[Frog]

The drawback of your measuring method is an underexposed screen surrounding the image

I'm not sure I understand.  If I take my standard art size of around 13x19, centered on a 23x31 screen, the math adds up to a 40" diagonal if the screens are touching. (13+5)*2 as the bottom "side" of the triangle, so 36^2 + 19^2 = c^2, where c = ~40, then x1.5 = 60 inches away.  That should result in fairly even exposure.

Al's math is a little off as well.  The diagonal on 2 23x31's next to each other (46x31 together) is 55"

My diagonal length  is based on the size of the frames, not the size of the image(s)
To fully expose the entire mesh area, without a hot spot in the center, I'm using the further distance.

Now, with photopolymers and careful developing, you could probably do okay with the smaller area and a post exposure to the entire frame, but without vacuuming or blotting, you may be increasing the risk of scumming.

[mimosatexas]

at 60 inches away the difference in distance from the very center to the outer corner is about 8%, at 90 inches it is just under 5%.  I'm not sure that 3% would make too much of a difference, especially when you factor in that the actual emulsion is a few inches in from the outer edge of the frame making that difference just under 2%, but with exposure times of about half.

I'm just starting to do this too though, so I want to use the proper distance as well.  Been crunching numbers here and there and it seems like any distance where the variance across the entire emulsion is less than 10% wouldn't lead to issues, but I am by no means an expert.

[Frog]

And remember, that units like the Olec heads have different reflectors available. They have a wide angle one that knocks the distance down to something like 90% of the diagonal. They are marked on the boilerplate.

[ZooCity]

Take the diagonal of the area you actually need to shoot, i.e., two screens, or four screens up as they would be placed in normal production or the biggest shop screen, whichever is larger.  Then look that sh!t up in the manual for your unit.  MH units have different reflectors and different optimal distances and the mfg or the manual should have information for you on what the multiplier is for each reflector type.  If you use a rule of thumb like 1.5, sure, it will work but it won't be best for detail and quality and consistency of exposure.

Also, I've learned that reflectors need periodic replacement.  Look into "super wide" reflectors for faster expo over large areas, it helps knock down the effects of the inverse square law.  I have one for our unit, just haven't installed yet.

[Orion]

Dan, you have an in with your comrades at NuArc, bet they could answer precisely...

[alan802]

I am about to start doing this due to picking up a huge vacuum frame, and logically for me it would be 1.5x the diagonal of the combined art on both screens.  I would also think it would be good to have the screens rotated 180 degrees so the light source is mimicking hitting both screens from the same angle/side.  See mockup.  distance would be 1.5x the red line, centered.

Did you pick up that ridiculously big but cheap one on Craigslist a month or so ago?  It was at a sign maker's shop downtown and he was moving and didn't want to take it with him.  I was on my way down to pick it up when our pressure washer and our AC for the sales offices went down at the same time and obviously didn't make it down to pick up.

[RonH]

I need that old "rule of thumb" info where you x your screen frame size by X to get your light source (distance) for two screens at a time? 23x31's  This is for a wall mount exp.  I think I recall 40" but that can change based on the screen frame size. Here, we are doing two 23x31's placed horizontal or side by side.

Anyone remember what that is?

Dan, you have an in with your comrades at NuArc, bet they could answer precisely...

The 1.5 times the diagonal rule came from the days of Carbon Arc light sources, and yes I was with NuArc when we manufactured Carbon Arc Units.  The reflector on carbon arc units was as much of an air deflector as it was a light reflector, so as not to affect the light output from the burning carbons.  Over the years reflector designs changed to get better coverage at different distances.  As an example our Trilight unit that has been on the market for around 15 years and will handle up to a maximum of a 42"x60" screen.  The glass to lamp distance on the Trilight is 31". 

As was already mentioned, increasing the lamp to glass distance will greatly increase the exposure time, which was one of the main reasons for the reflectors that we use today.  There is nothing wrong with the 1.5 rule, and if you do increase the distance you will get a little better coverage, but the down side is that it will take longer to expose the screen.

Ron Hopkins
NuArc Sales Mgr.
M&R Sales and Service Co.

[bimmridder]

Don't forget the Inversion Square Law if you change your distance from screen to lamp