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HouLite UV Exposure Light – UPDATE

The search goes on – for the perfect UV exposure unit. I’m not sure I’ve found it yet but I did actually spend money on something that I felt could come close; An LED flood light. What caught my eye was the wavelength in the UV-A range 385-405 nm. That is pretty close to the optimum wavelength for alternative processes, so I decided to give it a try.

My original plan was to construct an array of F20T8-BL fluorescent lamps. I picked up some fixtures that were on sale at the local building supply store. They were cheap enough, but the lamps cost so much I had to shelve the project until prices fell to a point I could afford them – yeah right. Needless to say that hasn’t happened and probably never will due to the dying demand for fluorescent lighting.

Breaking News – UPDATE

I liked the 20W unit so much, I upgraded to the 50W version! Sorry, I just had to say that. More on that later.

LEDs Are the Future

On the other hand I finally see production of LEDs strengthening across the board to the point that even the less common UV kind are coming down in price. This unit, purchased from Amazon cost under $30. It is small, light weight, consumes very little power and generates even less heat than a single 20 W fluorescent lamp and ballast. And much, much less than a 100 W metal halide lamp and ballast.

The unit I received is rated at 20 W and is constructed fairly well (except for the paint job). It includes a grounded AC cord and steel mounting bracket and has a good size heat sink which didn’t get too hot even after leaving the unit on for over an hour.

Test Setup

I attached the unit to my Minolta color enlarger so that the LED emitter was about 9.5 inches above my print and pointing straight down. This is the distance I visually judged would cover my entire 9×12 printing frame. I plugged it into a old style Graylab timer and picked out a 4×5 inch film negative set on top of a small sheet of  pre-coated SunPrints cyanotype paper. I locked the frame and started with a 5 minute exposure, then worked my way up until the highlights were clearly over exposed at 20 minutes. I washed each print in room temperature tap water for 20 minutes, followed by a quick dip in dilute peroxide bath, then washed again and allowed to dry over night.

For comparison, I made a 5 minute exposure in late afternoon direct sunlight.

The Results

Photograph – Council Overhang, Starved Rock State Park, near Ottawa, Illinois by the author

Row 1 top L to R – 5 minutes sun, 5 minutes UV lamp, 10 minutes UV lamp
Row 2 bottom L to R – 15 minutes UV lamp, 20 minutes UV lamp, 25 minutes UV lamp

Bottom Line

Pros – Easy to use, cost effective, good results in limited testing. Small (~1 inch square) emitter is compact light source. This may result in sharper prints than a light source that’s spread out over a larger area.

Cons – I noticed an odd problem when I used my Graylab 545 digital timer. The lamp periodically blinked off then back on. This only happened with the digital timer and could be a problem with the timer, not the lamp. However, it doesn’t happen with either of my incandescent enlargers. Also, I got a unit with a bad paint job, maybe you’ll have better luck.

Next step up is HouLight’s 50W unit which could does reduce exposure time by about 50% – yeah! – and increases coverage area too. Of course it costs more than the 20 W unit but still a bargain compared to commercial exposure units.

More Power

As Tim the toolman says more power is more better. It’s true. I’ve been printing with the 50W unit now for a couple of weeks. It’s not surprising that the results are right in line with expectataions. Especially nice for my classic cyanotypes which took up to an hour with the 20W unit, the new lamp is producing nice prints in half that time.

Troubleshooting Hints

If you are having a hard time getting the results you expect from your printing efforts and suspect that your contact printing frame may be a part of the problem, here are some ideas that may help. Keep in mind that I’m not an expert printer, but I do have some education and a little experience in the craft. I’m happy to help any way I can.

I appreciate comments on any of my posts, but if you have a specific question you’re probably better off using the contact form since I watch my email more closely than the comments.

The Frame

Well, it’s not rocket science. The four legs of the printing frame make up the basic structure and are what holds everything together. My frames are made of hardwood held together with finger joints and hardwood dowel pins at the corners. It is unlikely the frame itself will ever fail.

Glass

I use non glare glass in all my frames since about 2010. It is intended to reduce the occurrence of Newton’s rings.

“Newton’s rings is a phenomenon in which an interference pattern is created by the reflection of light between two surfaces—a spherical surface and an adjacent touching flat surface. It is named for Isaac Newton, who first studied the effect in 1717.” Wikipedia

Non-glare glass must be installed so that the etched side is facing the interior of the frame. That is, the etched side of the glass must be in contact with the non-emulsion side of the negative. If you can’t tell which side of the glass is the etched side, look at a reflection in it. One side will show a clear reflection, while the etched side will show a very dull reflection. If the glass is placed in the frame upside-down, your print will not look sharp.

It is also important that your negative is placed correctly in the printing frame too. The ink side ( we used to refer to this as the emulsion side) of the negative should contact the printing paper. Again, if the negative is upside-down your print will not be as sharp.

Split Back Springs Feel Loose

If the back feels loose here is how you can do the tighten up. I attach the springs using a screw instead of a rivet like some other frames. Like everything this has its upside and downside. The upside is you can remove the springs and give them a nice polish with a little Brasso or other metal cleaner, assuming of course you have brass springs. Wooden springs can be cleaned up with a light sanding. Downside? Yes, the screws can get loose and make the frame feel a litttle woogity. The fix is easy. You don’t even need a screwdriver. Just spin the spring clockwise till the screw is tight again.

Broken Glass

Glass can break – duh. If you experience this misfortune, don’t despair, but be careful picking up the pieces. I can replace your glass with the original non-glass type I used when I made the frame or you can obtain replacement glass from a local provider. I recommend 2.3mm (1/16″) non-glare picture frame glass. Other dimensions vary with the size of frame. Best bet is to take the frame to the glass shop and ask for a piece that fits easily in the frame. Also has smooth edges so you don’t get cut when you remove the glass for cleaning.

If you have to order a replacement from me (or anyone else), please send the exact inside dimensions to the nearest 1/32″ of the frames width and length when ordering. Include your address and I’ll give you a quote including price and shipping.

Normal Wear and Tear

I think it would take a lot of work to wear the finish off of one of my printing frames. However there is one place that will show wear before any other. That is where the springs rub against the back when locking it in place. To repair the rub marks, remove the back from the frame and remove the springs from the back. Use 120 to 320 grit sandpaper to prepare the surface and apply two or three coats of Minwax High Gloss Polyurethane varnish. For best results allow varnish to dry completely and sand lightly or buff with 00 steel wool between coats. Reassemble springs and enjoy. Note: Don’t remove hinges unless absolutely necessary since the threads made by wood screws can be stripped easily.

Distance and Exposure

The distance from the light source to your print is important when determining exposure time. I mounted my 20W UVLED exposure unit under the lens holder on my Minolta enlarger so I can easily adjust the height. I can move it up to cover a large print and down for a small one. The advantage of moving the light closer to the print is reduced exposure time. But just how much should I reduce the exposure time, you might ask. Well, here’s how to figure it out. It’s based on the principle that the intensity of light is proportional to the square of its distance from the subject. The inverse square law, IL=1/D2 applies here but keep in mind it’s relative.

For example, if your light is 10 inches from your print it would have 1/100th the intensity as if it were 1 inch away. That’s pretty simple but what happens when we have to deal with real world measurements. Let’s say you were exposing a print with the light source 18.5 inches above the print and you measured the standard exposure at that distance to be 68 minutes. Then you realize that you don’t need to cover such a wide area and you can move the light closer to the print. It looks like you’re getting pretty good illumination when the light is only 11 inches above the print, so how much is the new exposure time? We’ll calculate the exposure factor, a value we’ll call EF, by which you can divide the original exposure time to obtain the new exposure time.

Formula: EF = E12 / E22

Where: EF is the exposure factor, E1 is your original distance and E2 is the new distance

E1: 18.52 = 342.25
E2: 112 = 121
EF = 342.25 / 121 = 2.83

Divide the original exposure time (68 min) by 2.83. That’s about 24 min. See you just saved yourself a bunch of time. If you’re moving the light source away from the print, simply multiply the original exposure time by the exposure factor instead of dividing.