Summary: In this follow-up post to our Characteristics of Video Data for New Users post, we’ll highlight some of the important lighting factors to understand so that you can get the best data from your collections.
Lighting
Proper lighting is crucial for collecting clear video data. In general, natural lighting provides excellent illumination of subjects for markerless motion capture, but there are important considerations to be made when relying on natural lighting. For example, be aware of the position of the sun, avoid lighting that provides harsh back-lighting of the subject, and take care to adjust aperture, exposure, and ISO/gain settings to prevent washed out or dark images. Conversely, indoor environments typically have lower ambient light levels and the concern here is lack of light. Finding a balance of the correct video and camera settings can be more challenging indoors, but can be managed during pilot testing.
Even indoors it is important to consider natural sources of light, especially windows or doorways. On cloudy days, for example, natural light can be quite a bit brighter than indoor lighting, and windows or doors provide light that is often at the same height as the participants and cameras. This can lead to backlighting of the person, which creates silhouette-like effects with minimal detail visible on the person. Being aware of all lighting sources and their direction can help you adjust your camera setup or settings to best capture the participant.
Large windows can provide considerable backlight, turning the subject into a silhouette.
Intensity
Light intensity, or more simply, the brightness of the incident light, is measured in lumens or lux (lumens per square meter). This is one of the most important factors when considering the lighting available to you, and can vary greatly; outdoors, natural light can easily fluctuate between 1,000 and 100,000 lux depending on if the light is direct or shade. Indoors, typical lux values fall between 100 and 2,000 lux. For markerless motion capture, the minimum light intensity should be considered to be 500 lux, but an intensity of 1,000 or more is recommended!
The ‘brightness’ of light in a certain area depends on the luminous flux of the light source, measured in lumens, and the area over which the light falls. Together, these convey illuminance, or the amount of incident light on a surface, measured using lux (lumens per square meter).
Color Temperature
This describes the hue of the emitted light, and is typically expressed in Kelvin. A ‘white’ light source can vary drastically in their color temperature between warm yellow/orange hues to cool blue hues, a change that corresponds to increases in temperature. For markerless motion capture environments, a light source emitting a neutral to bright, cool white light in the range of 3500K to 6500K is ideal.
This image shows how artificial light sources with different color temperatures provide varying degrees of cool vs. warm light.
Color Rendering Index
Color rendering index (CRI) is a measure of the color accuracy of objects illuminated by a light source. CRI is typically expressed as a value from 0 to 100, with higher values providing more accurate color rendering. In order to capture well balanced videos, it is important that the colors in the images accurately reflect those in reality, and so using lighting with a color temperature in the suggested range and with a high CRI will be beneficial. CRI values for modern lights do not vary significantly, and most will fall in an acceptable range of 70+.
Directionality and Uniformity
The directionality and uniformity of light can play a significant role in determining the overall quality of the lighting within an environment. For markerless motion capture, diffuse, non-directional light is often better than directed light due to more even lighting and shadows, and a reduced potential for glare off of surfaces. However, direct light can also provide excellent and even lighting if it is provided from all directions. It is still important to avoid ‘hotspots’ which can lead to parts of the image or person being overexposed or washed out while the rest of the image is properly exposed.
The images above demonstrate differences in lighting uniformity, with the first showing more uniform, diffuse lighting and the second showing more direct, concentrated lighting. Diffuse light generally provides more even lighting throughout the volume and more even lighting of the subject from all sides. Direct lighting can lead to ‘hotspots’ and inconsistent exposure of the participant as they pass throughout the volume.
Flicker and Frequency
Modern light bulbs come in many different varieties that operate differently, some of which have an inherent flicker or illumination frequency associated with their function, such as fluorescent lights. In general, it is best to avoid lighting with flickering at or around the frequency of the videos being recorded. For typical fluorescent lighting, this is 100-120 Hz depending on the AC power frequency. LED, halogen, or fluorescent tubes with electronic ballasts are some bulb options with reduced or no flicker, which will provide more even lighting when capturing high speed video data.
Lights that flicker at a similar frequency as the video frame rate can lead to inconsistent image brightness and may lead to over- or under-exposure of the participant, or inadequate image brightness during some frames.
Functional lights flicker at a much higher rate - so high that it can be barely perceivable unless captured by cameras at high frame rates. You can think of it as a fan blade moving quickly; it's hard to see the blades individually until you take a picture.
Conclusion: We hope these two blog posts on video data characteristics help you to understand the many aspects of video data that are important to consider when preparing your markerless motion capture data collection!
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