Exposure Time
Introduction
The exposure time, also known as shutter speed, is the amount of time that a single camera image is exposed to light. In other words, for how long the shutter remains open. A Zivid camera takes multiple images in order to calculate depth, and the exposure time of an individual image is defined in microseconds from 1 677 to 100 000. The minimum exposure time is camera dependent:
Zivid One+ |
Zivid Two |
|
Minimum exposure time (µs) |
6 500 |
1 677 |
When exposure of 6 500 µs is chosen, the Zivid camera needs about 90 ms to capture a 3D image. Increasing the exposure time by some ratio will also increase the exposure of the image by the same amount. For example, if you increase exposure time from 10 000 µs to 20 000 µs, the acquisition time will be twice as long. The light intensity will be twice as high or increased by 1 stop.
Interference from ambient light sources
When taking images with a Zivid camera, we are sampling light information in a certain time window. By utilizing this simple concept, it is possible to avoid many issues that can arise from ambient light.
Structured light sensors are sampling systems that operate with similar sampling rates as many utilities such as TVs and lighting. A fluorescent lamp typically connects to the power line, which is an AC power source. In the EU this power source produces electrical voltage as a sine with a frequency of 50 Hz. In other countries, such as the US, the power line frequency is 60 Hz.
Consider a video camera with a frame rate of 30 fps. Direct it at a fluorescent lamp in Europe and hit record. You will find that there appears to be flickering in the video. You have probably experienced this phenomenon when filming with a smartphone inside a garage or warehouse.
Now consider a video camera where the capture rate matches the frequency of the ambient light source. You would end up sampling light with the same phase in each image, and the flickering would appear to be gone. This would make the light appear constant.
The explained concept is called correlated sampling and has favorable effects in structured light 3D scanners. It enables to filter out noise that varies with time from an ambient light source by choosing an exposure time that satisfies the equation below:
where \(n\) is a positive integer, and \(f_s\) is the frequency of the light source. For this reason, it is recommended to use sampling rates in the multiples of the grid period.
Multiples of 10 000 µs in the presence of 50 Hz grid frequency.
Multiples of 8 333 µs in the presence of 60 Hz grid frequency.
Light sources that have DC power sources, such as spotlights, or LEDs with ≥1kHz will not cause noise that varies with time.
50 Hz
The table below shows the recommended exposure times to use in countries that use 50 Hz power line frequencies. Be aware that you can also use other multiples of 10 000 µs, such as 30 000 µs and 50 000 µs.
Exposure time (µs) |
10 000 |
20 000 |
40 000 |
80 000 |
100 000 |
Capture time (ms) |
130 |
250 |
490 |
970 |
1 210 |
Stops |
0 |
+1 |
+2 |
+3 |
+3.32 |
Tip
In the presence of 50 Hz light sources, it is recommended to an exposure time that consists of multiples of 10 000 µs.
60 Hz
For countries in the Americas, such as United States, Canada, and Brazil, as well as parts of Asia, e.g. Japan, South Korea, and Taiwan, and exposure time based on a power line frequency of 60 Hz should be used as shown in the table below:
Exposure time (µs) |
83 33 |
16 667 |
33 333 |
66 667 |
83 333 |
Capture time (ms) |
108 |
217 |
433 |
867 |
1 083 |
Stops |
0 |
+1 |
+2 |
+3 |
+3.32 |
Tip
In the presence of 60 Hz light sources, it is recommended to an exposure time that consists of multiples of 8 333 µs.