uV LED lightbox - Page 1
Forum section: electronics / projects

author: Davy
author_rank: Regular Contributor
timestamp: May 09, 2019, 07:19:18 pm
content: I have loads of 5mm ultra bright uV leds, the viewing angle is 20deg. wondering if 15mm led-to-led spacing would be OK to avoid dead spots or shadowing in a uV exposure box?Dave

author: OM222O
author_rank: Frequent Contributor
timestamp: May 09, 2019, 07:46:54 pm
content: you didn't mention anything regarding the power dissipation. that's the most important thing, so if you have enough heat sinking or active cooling, it should be fine.

author: ajb
author_rank: Super Contributor
timestamp: May 09, 2019, 08:19:48 pm
content: Depends on distance and the array shape. It also depends on the field distribution of the LEDs, depending on how even of a distribution you need for your application. For a rectangular array at 15mm center-to-center spacing you need the projected spot size to be at least 22mm in diameter to avoid a dead spot in the middle. For a hexagonal array you need a 15mm spot. Some trigonometry based on the 20mm viewing angle will give you the distance you need between the LEDs and the projection surface. Realistically, the output of the LED doesn't drop to zero immediately past that 20 degree angle, but the exact shape of the dropoff depends on the LED. The datasheet hopefully has a polar graph that shows the approximate field shape.

author: sleemanj
author_rank: Super Contributor
timestamp: May 09, 2019, 10:13:08 pm
content: The wider you space the leds, the taller you need the box to be. With a 15mm spacing your box should be around 45-50mm from the emitter to the artwork.However, don't be afraid to use a diffuser. I've made a number of UV boxes and used diffusers in most of them, usually with one diffuser layer halfway between LEDS and glass, and sometimes another diffuser directly on the bottom of the glass. You can still do plenty fine enough traces no trouble with diffuse light, I rather have an even but diffuse lighting than have hot-spots where every led is.

author: Davy
author_rank: Regular Contributor
timestamp: May 09, 2019, 11:57:56 pm
content: Thank you to you all for the response. These are through hole domed types, too close and I get good spotlights, the distance from the LED's to the 'target' seems to be around 40mm when trying three LED's lashed up, think I'll lash a couple of banks up on veroboard (I hate that stuff) and have a play, I've a feeling 10mm spacing might be an overkill and need have a play here.The LED spec-:3000mcd.3.2 -3.8V @ 20-30mAViewing angle is 25 deg, not 20 deg. as I said before.I read somewhere about glass being opaque to uV, I'll have a dig round later. I replaced bulbs in a 'S' meter once using two white SMD LEDs and used a diffuser from a flat panel screen to stop the glare, it worked a treat, the stuff looked like frosted Mylar.... I need do some playing here.Dave

author: sleemanj
author_rank: Super Contributor
timestamp: May 10, 2019, 12:23:58 am
content: > I read somewhere about glass being opaque to uV, I'll have a dig round laterGlass blocks most UVB, but only a little of UVA. We are only dealing with UVA so most of it gets through just fine.I have used either glass from scanners (well, multifunction printers, which can be usually bought for a dollar or two secondhand) for large boxes, and my current mini-box uses glass out of a cheap $2 picture frame.

author: wilfred
author_rank: Super Contributor
timestamp: May 10, 2019, 02:16:47 am
content: I had a half finished UV LED lightbox so your post prompted me to dig it out. It uses 99 LEDs from ebay in 33 strings of 3 LEDs arranged in an 11cm x 9cm grid. Which makes the gaps about 1cm give or take a mm.Since I have my camera out for another post I decided to do a quick test. I suspended a sheet of tracing paper over the array at heights of 5cm 8cm and 12 cm. I tried the camera on AUTO for pictures 162 (12cm), 163 (8cm), and 164 (5cm). Pictures 167 (5cm), 168 (8cm) and 169 (12cm) were taken with the camera set on manual exposure and ramp right down to see if that helped reveal the evenness (or not) of the light pattern.One thing I can notice is that the LEDS are not perfectly perpendicular to the board and that makes a difference at 5cm (164, 167). Another thing to consider is the camera is pretty close to the paper and that means the center LEDS point into the lens directly more than the edge ones. that isn't an issue for a PCB. The screw driver was to help the camera focus.The screenshot is to make side by side comparison a bit easier. There's a 2MB limit on photos per post.

author: wilfred
author_rank: Super Contributor
timestamp: May 10, 2019, 02:26:20 am
content: This is how I made it on a piece of stripboard. It was easier to use the component leads as wires. 33 strips of 3 LEDs and a 100 Ohm resister. 12V and it draws about 1 Amp.

author: Davy
author_rank: Regular Contributor
timestamp: May 10, 2019, 09:58:33 am
content: Wow, a lot to go on, many thanks.My plan was to use a flatbed scanner, 15mm apart keeps nagging me and don't know why, trigonometry not my strong point here, 10mm centre-to centre seems too close, beginning to think I should half that distance as wilfred indicated.I did expose a pcb once in Sunshine, was just before mid-day about 11.30 around May or June... I let it stand for 8 minutes it turned out perfect and I used inkjet film with two copies taped together AND I did use a glass plate to keep the film flat so glass was no problem there, I was quite surprised it worked, no trace problems or nothing, going back a few years now mind.