Ambient Light Robustness#
Which Type of Ambient Light Is a Problem?#
The blaze-101 camera emits invisible infrared light (940 nm). Invisible infrared light in the 920–970 nm range is detected by the camera's image sensor and interferes with the camera's own light source, thus affecting the resulting measurements. It can therefore be considered the main cause of disturbance. The effect of light of other wavelengths is negligible in this regard.
Irradiance vs. Illuminance#
Because of the camera mainly being susceptible to invisible infrared light, the ambient light robustness is specified in terms of irradiance, a radiometric quantity suited for invisible light. Irradiance is measured in W/m². It refers to the light in terms of its radiant flux (power) that is received by a surface per unit area.
In contrast to this, the perceived brightness of visible light is usually expressed as illuminance. This is a photometric quantity that refers to light in terms of its perceived brightness to the human eye. Illuminance is measured in lux (lx).
There is no universally valid way to derive illuminance values from irradiance values. This is because the infrared part of different light sources can vary significantly and, therefore, the spectrum of each light source has to be considered individually.
Comparing Sample Scenarios#
To offer some guidance, Basler has examined three different scenarios to show how irradiance relates to the illuminance values of different light sources. The following conditions were applied:
- The blaze camera's irradiance limit is 12.8 W/m² (determined by Basler under these test conditions).
- The total spectral irradiance of typical light sources, e.g., sunlight or halogen light, was chosen in such a way that the irradiance registered by the camera's image sensor was equal to the camera's irradiance limit of 12.8 W/m².
The illuminance in the visible light range under these conditions was then calculated taking into account the spectrum of the light source and the luminous efficiency function. Also included were the quantum efficiency of the image sensor, the transfer function of the camera's bandpass filter (passband and stopband), and some other relevant camera properties.
The following table lists the results:
|Scenario||Radiometric Limit for Ambient Light Robustness |
(max. Irradiance @ 920–970 nm)
|Photometric Limit for Ambient Light Robustness |
(max. Illuminance, Approximation)
|Sunlighta||12.8 W/m²||~60 klux|
|Incandescent light (3200 K)b||12.8 W/m²||~8 klux|
|Artificial light sources without near-IR emission between 920–970 nm (e.g., visible LED, fluorescent lamp, etc.)||12.8 W/m²||No practical limit|
Bright, sunny day with blue sky in the northern hemisphere: day of summer solstice; solar time 12:00:00; latitude 53.692540; longitude 10.266444; target tilt 90 ° (perpendicular to ground); target azimuth 180 ° (south-facing); surface albedo 0.2; camera normal to target
Tungsten-halogen lamp, 3200 K color temperature
Effects of Ambient Light#
The effects of ambient light on the camera's image quality can be observed best in the range map or point cloud. In scenes with strong ambient light in the 920–970 nm range, the depth noise increases, thus reducing the precision of the depth measurement.
If the ambient light is relatively weak (irradiance up to 5 W/m²), the increase in noise remains below a factor of about 2. Under strong ambient light, close to the specified limit of daylight robustness (irradiance 12.8 W/m²), the noise increases by a factor of 7. If the irradiance of the ambient light is increased further beyond this limit, the camera's image sensor saturates. When this happens, the intensity image appears white and depth measurements are no longer possible.
Ambient light isn't visible in the camera's intensity image. This means that ambient light changes can't be observed as brightness changes but only as changes in the image noise. The transition from a well-exposed image to an oversaturated image may therefore appear very sudden.
Determining the Irradiance Limit of the Camera#
The camera was tested under the following conditions:
- Flat, white target with 90 % reflectivity
- Target illuminated by 940 nm LED
- 6 m distance between target and camera
- Exposure time of 250 µs
The camera was pointed directly at a flat, white target at a distance of 6 m. To simulate ambient light, the target was illuminated by an additional light source using the same wavelength as the camera. The irradiance on the target was increased up to the point where the camera still delivered usable images without overexposure. The irradiance was measured and the increase in noise was determined based on the camera images.
The camera still delivered usable images at an ambient irradiance of 12.8 W/m² for ambient light sources between 920–970 nm. The image noise increased by a factor of 7.