How to Measure IR IlluminationBy John Honovich, Published Feb 15, 2012, 12:00am EST
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 [link no longer available]. 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.
In our field testing, the optimal range of IR light or producing clear evenly exposed surveillance video is about 1-5. When measuring IR, the metric used is microwatts/centimeter squared or uW/cm2. As such, this is 2-5 uW/cm2.
If you have substantially more than 5 uW/cm2, the result will likely be an overexposed image. Think of the illuminator like a car's headlamp. The IR illuminator is typically a narrowly focused beam used in an overall dark area. When the beam is too powerful for the camera, the result is overexposed video.
The exact amount of light that is optimal for a camera depends on a number of factors:
- The amount of visible light (i.e., is it 0.1 lux or 0.5 lux visible light - more visible light, less IR needed)
- The visible light sensitivity of a camera (i.e., does the camera 'good' or 'bad' at low light in general)
- The camera's responsiveness to IR (which might vary modestly among D/N cameras)
For instance, cameras that performed the best in our low light shootout would need less IR light (closer to 2) while those that did poorly would need more (closer to 5).
Practical Uses of Measuring IR
In our testing and usage, we found a number of practical benefits of measuring IR illumination:
- Benchmarking camera performance:
- Benchmarking IR illuminator performance: Knowing exactly how much IR illumination is generated at a specific distance is more useful than just a manufacturer specified range. For instance, if you test an illuminator and power output at 100 feet is only 1.1uW/cm2 that tells much more than a 150 feet range specification.
- Adjustment: Because IR light is invisible, it is hard to know when it is optimally aligned. You can view video on a monitor but we found it far more accurate to take IR readings as we adjusted the illuminator (both vertical and horizontally).
- Maintenance: Over time, image quality may degrade. Using a meter can help determine if the IR illuminator simply was shifted out of alignment or if it is a result of a decrease in power output from the illuminator as the LEDs/illuminator ages.
In the video below, we demonstrate how the basics on how to measure IR illumination:
In this video, we look at how to setup the meter including specifying the correct illumination range to measure:
Image Samples with Measurements
Take a look at the image below for a sample IR measurement. The reading is 7 uW/cm2 at 30 feet from the camera and IR illuminator. The subject is clearly overexposed as there is too much IR illumination for this camera's light sensitivity and the scene's light level. Note: that not all cameras will be overexposed at 7 uW/cm2. For instance, the camera used here is the Bosch 1080p which is strong in low light even without IR. By contrast, an Arecont 5MP might need 7 uW/cm2 or even greater IR illumination.
Now, we have the same camera with the subject and measurement at 100 feet. Here the IR illumination is down to nearly 1 uW/cm2. Unlike the close up shot, our subject is fairly evenly illuminated. The practical problem is that the FoV is too wide to capture much details. Of course, this could be adjusted by narrowing the camera's FoV.
Example of Varying IR Power
Some camera manufacturers have recently started promoting cameras with built in IR that automatically adjust their output power. This addresses the overexposure problem we showed in the last section. In this Samsung demo video, it shows how the camera dynamically adjusts the IR power output to optimize the power level for the subject's location in the scene.
We do not know how well this works and are showing this primarily to reinforce the important of optimizing the output power to ensure proper exposure of subjects when using IR.
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