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I am performing an experiment in a lab, which requires taking time-lapse footage of some samples in an incubator oven. The oven does not have a built-in light source, so this question is about how to set up the lighting.

The set-up is as follows: the samples are arranged on a rack at the back of the incubator, which is not much bigger than a large kitchen oven - the inside is between about 40 and 60 centimetres in each dimension. The sample rack takes up basically the whole back wall of the oven, and we need it to be lit evenly. The samples are not photosensitive, but we need to get a decent image with the relatively cheap camera, and it's important to get good colour reproduction.

The oven door has a smallish double-glazed window, and we'll be photographing through the glass, using a webcam type of camera that has an optical zoom, taking still images periodically using a Raspberry Pi. The reason for shooting from outside is that the inside of the oven will be 80 celsius, so I worry about the ability of a camera to survive inside the oven in the long term. It would also put the lens a bit too close to the samples, making the ones at the edges hard to see even with a very wide lens, due to the perspective.

For lighting I guess we have two options:

Option 1: Lights outside the oven. This was our original plan. The idea is to put two diffusers (probably just white card) on either side of the oven, and aim directional lights at those, so that the reflected light illuminates the samples. However, from testing this it was quite hard to avoid glare from the double glazed window, or reflections of the camera itself. It was also harder than I imagined to light the whole sample rack evenly.

Option 2: Lights inside the oven. This seems ideal given the issues above - if we can set up the lights inside the oven itself then we can avoid reflections much more easily. However, I don't know if there is any light source that is suitable for this. It would need to be able to survive 80 degrees for a sustained period of time (likely upwards of a year), and it would also need to not give off much heat itself, or it will overheat the oven. It would also need to give off quite diffuse white light. As a more minor issue, the cable would be quite thin in order to be able to seal the door.

Given this, my questions are:

  1. is option 2 feasible/sensible? If so, what kind of light source would work for this, and what's the best way to get the lighting nice and even in such a confined space?

  2. if option 2 is not feasible, what's the best way to light the samples from outside the oven, while avoiding glare or reflections? (Is there a better way than the one I suggested?)

(Note: I don't have the dimensions of the oven or its window to hand, but I will edit them into the question later.)

  • A lot hinges on whether the process needs to be totally automated or if the images can be taken manually at the needed intervals. Which is it? – Michael C Feb 2 '18 at 22:16
  • @MichaelClark it's automated - we'll take one photo every hour or so for about a year. – Nathaniel Feb 3 '18 at 0:12
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I don't think you're going to find a good light source that will withstand those conditions — at least not one that is meant for general photography rather than lab equipment. That means the problem reduces to option two. We already have several questions about that, like How can I take pictures through a skyscraper, airplane, or train window?.

Specifically, I'd recommend using a polarizing filter to cut the glare. It is likely that simply using such a filter in the right orientation will be sufficient — but if not, you can also use polarizing gels on the light source itself, oriented the other way.

  • Any old-style incandescent bulb can withstand 80ºC. – Michael C Feb 2 '18 at 22:09
  • Yeah, I guess I was mostly thinking about putting a flash or studio strobe under those conditions. My Godox units say they're not to be stored in conditions hotter than 50C. – mattdm Feb 2 '18 at 22:14
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Option 2: Lights inside the oven....It would need to be able to survive 80 degrees for a sustained period of time (likely upwards of a year), and it would also need to not give off much heat itself, or it will overheat the oven. It would also need to give off quite diffuse white light. As a more minor issue, the cable would be quite thin in order to be able to seal the door.

LED sounds like the way to go. They use little power and give off relatively little heat, they have a long operating lifetime, and they're available in a wide range of color temperatures. 80°C is well within the operating and storage temperature ranges for many LED units. The individual parts don't need much space and range in price from a few pennies to a few dollars each, so you could build an array of them covered with some sort of diffuser to get the soft, even light that you're looking for. You can also buy pre-assembled panels of LEDs which might suit your purpose.

Incandescent lamps would also work fine at 80°C -- the filament in an incandescent bulb operates at around 2500°C, and incandescent bulbs are used in millions of home ovens around the world. The heat given off by the lamp would presumably be compensated for by the oven's thermostat. That is, with an incandescent bulb contributing heat to the oven, the main heat source just wouldn't have to run as often. Incandescents do have a shorter lifetime, so if you can't open the oven at all in order to replace the bulb, that might be a problem. Look for "appliance rated" lamps, which seem to be designed for higher temperature environments.

Option 1: Lights outside the oven....However, from testing this it was quite hard to avoid glare from the double glazed window, or reflections of the camera itself. It was also harder than I imagined to light the whole sample rack evenly.

A ring flash or twin flash could work well here. A twin lite would be particularly effective because you can adjust the angles of the two lights. The light might not be as diffuse as you're looking for, but with light coming from both sides of the lens shadows shouldn't be a problem. Putting the camera lens close to the surface of the window should help avoid glare.

  • Plus you could only power the LEDs when you take the picture. – Eric Shain Feb 2 '18 at 19:05
  • I checked the datasheets for several (at random) of the LED units you linked and some of them note a maximum ambient temperature of 85C. That's over 80C, of course, but I wouldn't say "well within", and so at the very least you should shop carefully if you go that route and you may need some system for airflow so you don't get a pocket of hotter-than-overall air around the lights. I bet their lifetime won't be so great in these conditions. – mattdm Feb 2 '18 at 22:23
  • @mattdm With tens of thousands of different products on the market, I didn't want to recommend anything in particular -- the OP really would need to do some research, as you say. But here's a data sheet for some Luxeon products(PDF) that says "Operating Case Temperature" is -40°C-105°C, and I'd call 80°C "well within" that range. I'm not cherry-picking here -- I found a number of products that say the same. The important point is that LEDs can work at that temp, but of course some will work better than others. – Caleb Feb 2 '18 at 22:33

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