Using IR for night time surveillance is a popular choice. The added non visible light can improve image quality when ambient street or visible lighting is absent. However, adding the right amount of IR light can be challenging. Add too much and the scene is over exposed. Add too little and the scene is still too dark. Measuring IR illumination is a useful way to get it just right and to identify invisible problems. In this report, we explain how to do so.
The Typical Approach
Typically, when you want to choose IR illumination, you look at a manufacturer's specification for maximum distance and beamwidth. The maximum distance indicates how far you can 'see' from the illuminator while the maximum beamwidth signifies how wide you can view (e.g., 10 degrees, 30 degrees, etc.)
The challenge in those numbers, like camera manufacturer minimum illumination ratings, are hard to use and compare. They are not standardized or vetted. The specified levels may not match actual performance plus comparing different manufacturers based on specs is unlikely to be accurate.
Day Time Approach
During the day or with conventional camera setups, if you wanted to measure light levels, you would use a lux meter (and you should use a lux meter - see our tutorial on using lux meters). Knowing the exact (visible) light levels can really help knowing how well a camera will work in a given scene.
Unfortunately, 'regular' lux meters are not designed to measure IR illumination. If you have an IR illuminator on in the dark and hold up your 'regular' lux meter to it, it will likely register 0 - an unhelpful and misleading result.
Specialized Meters to Measure IR
Specialized meters do exist that measure IR light. The big downside is that these options are much more expensive (thousands of dollars) and scientific oriented.
We did find one that was relatively inexpensive (~$500) and field usable - the Coherent LaserCheck. We bought one and did a test. Here is a sample image of us demonstrating it.
Inside, we provide two video screencasts showing how to use this meter in action and how to best optimize your use of IR illumination.