Illumination for high-speed imaging can be challenging because of the photographic trade-offs involved. Many high-speed applications require short exposures to eliminate motion blur and high f-stop settings to maximize depth of field. More “sensitive” cameras often have larger pixels, creating an even greater challenge with depth of field.
All of this often leaves us scrambling for more light. Adding lots of light, however, can bring its own problems, including needing more power, creating heat, making the environment uncomfortable for humans or animals involved, etc.
LED illumination has become popular for high-speed imaging in the last few years because of its efficiency (lumens/watt), economy (lumens/$), and availability of daylight-balanced LEDs.
There are a couple of other properties of LED illumination that make it especially attractive for strobing applications:
- Extremely fast “on” and “off” times allow LEDs to be strobed (turned on and off) at very high frequencies.
- Limiting a strobed LED’s duty cycle (% of “on” time) allows over-driving LEDs way past their steady-state power (and illumination) limits.
A strobed LED may produce pulses of high-intensity illumination at very high frequencies and very short pulse lengths, which is ideal for high-speed cameras! The LED is off most of the time, so it is not consuming a lot of power or creating a lot of heat. Limiting the duty cycle also lowers the perceived brightness to humans and animals.
One question that might come up is, “Can strobed illumination cause photosensitive seizures?” For high-speed video we are talking about frequencies from 250Hz to 10KHz, which is far from the 1Hz to 20Hz range that might cause seizures. Standard fluorescent bulbs strobe at 60Hz, for example, which is faster that human perception can detect.
The next thing you may be wondering is, “what is this going to look like?” The mention of strobe lights conjures up a few different things: 1) strobe lights on the dance floor making everyone appear to be moving in slow motion and 2) multi-flash strobe images like this one where there are multiple exposures on one still frame.
For high-speed imaging, there are several ways to utilize the strobe, but before listing them, let’s first nail down the concept.
For any given sensor, you will need some quantity of light to make a good image. We measure light quantity as “Intensity x Time”. In other words, some number of “lux x seconds” is required for a good image. If the light intensity (lux) is very high, the needed time can be very low. Sensitivity for image sensors is often quantified just that way: in Volts/lux/sec.
In the camera the exposure time is usually constrained by the maximum and minimum shutter duration possible for whatever frame rate the camera is operating at. The maximum exposure time is close to ‘1 /<frame rate>’. With digital sensors, you need to subtract a little time for the sensor to reset. Minimum exposure times are 3 microseconds or less.
The light intensity is constrained by the intensity of the light source and the lens f-stop required for adequate depth of field.
An alternative to setting a short shutter duration, however, is setting a short flash for the illumination. LED strobe illumination is often equipped with synchronization I/O signals, making it possible to flash the strobe one or more times for every frame taken by the camera.
Getting back to the question of what the video will look like. For most applications, you will see very clean video with minimal motion blur. It is also possible to use the strobe to create multi-exposure effects, even while capturing high-speed video. This video demonstrates some of the possibilities.
If you are interested in using strobed illumination with your Fastec camera, please contact us for more information.