Knowledge Base

Ideal Light Conditions

A good lighting setup is vital to get a good image that EthoVision can use to accurately track your animal. This is particularly important when you want to do nose-tail tracking or track multiple animals with color markers. To produce a good image, the lighting must satisfy three criteria:

• It must be bright enough. Stand-alone video cameras (which we recommend) can sometimes work at 0.1 lux or darker. At higher light levels you can work with a smaller aperture. See also Using EthoVision XT in near darkness. Please note that poor lighting gives more noise in the image which enlarges the video file size.
• It must be even. That is, a diffuse light source, such as a shaded fluorescent tube or globe-type incandescent bulb. Spotlights can produce changing shadows and reflections.
• There must be no reflection of the light source visible in the image (see below).

TIP Watch the video Set Up Your Test Environment, which you can find in the EthoVision XT video tutorial (Help > Video Tutorial).

Eliminating Reflections

If you have reflections in your image, EthoVision may confuse those reflections with your subject, and track the reflections rather than your animal. Preventing reflections is particularly important when you want to do nose-tail tracking. There are a number of measures that you can take to reduce this problem:

• Surfaces within the arena should be (whenever possible) a dark color and matte texture.
• Lighting should be from a diffuse source, such as a shaded fluorescent tube or globe-type incandescent bulb.
• The light source should not be in direct line-of-sight of the camera (that is, it should be indirect lighting). You can achieve this by bouncing the light off the ceiling or walls (which should be a pale color and matte texture) and, if necessary, placing a shield in between the light source and lens.
• If your arena is transparent (for example, a glass olfactometer), you can use backlighting. This will eliminate reflections, but you must take special care that the lighting is even and diffuse. Place the light source underneath the setup and attach a sheet of white paper to the bottom to create diffuse back light. It is important to avoid light coming from above as much as possible, otherwise there is no effect of back light.
• You could also try a polarizing filter in front of the camera lens. The filter will remove reflections that are incident to the reflective surface at 32°–37°.
• If possible, draw your arena in such a way that bright reflecting rims of mazes are excluded.
• To remove reflections brighter or darker than your animal, set appropriate thresholds in the Detection Settings.
• To remove reflections larger than your animal, enter an appropriate value for the Maximum Subject size in the Detection Settings. Note that if your animal enters the area of the reflection, EthoVision might no longer be able to track it if you use this setting.

Lighting a Morris Water Maze

Lighting a Morris water maze needs to be carried out particularly carefully because the water can give large and variable reflections. Some of these can be excluded by the software settings listed above, and the following lighting setup should give good results:

Place four (or more) bulbs round the pool, below the level of the water surface or at the ceiling. 'Globe' type bulbs are ideal (twice the diameter as standard incandescent light bulbs). They should be close enough to the pool wall so that there is no direct line of sight between the bulbs and the camera lens. The light is reflected off the walls and ceiling, so that it only reaches both the lens and water surface indirectly.

• Take special care when dealing with water and electricity in the same room. The lights should be placed so that water cannot splash on them, ideally they should be suitable for outdoor use (double-insulated) and if connected to an electrical outlet in the same room as the water maze they should be connected through a circuit-breaker.
• Ask a photographer for advice on the lighting of your water maze.

Lighting Water Tanks

When using water tanks like in the Porsolt forced swim test, a problem is often that there is a lot of reflection, especially just above the water surface. Furthermore, detection of the animal can be hindered by the water's meniscus.

Try to make a set-up with as much contrast between animals and background as possible. For instance, when you have white rats, preferably use a completely black background. Back light (that is, light from behind the subjects so you only see the animals' contour) generally gives very good results. A solution to remove the water's meniscus in the video image is also to place the test cylinders in a separate tank with water (for example an aquarium) and then make recordings. The water level in the tank should be higher than in the cylinders.

For more information on the Porsolt forced swim test, see the EthoVision XT 19 - Application Manual.

Source: EthoVision XT 19 Help, Noldus Information Technology