Ambient Light Robustness#
Which Type of Ambient Light Is a Problem?#
blaze cameras emit invisible infrared light:
- blaze-101 cameras emit light at a wavelength of 940 nm
- blaze-102 and blaze-112 cameras emit light at a wavelength of 850 nm
Invisible infrared light in the ranges of 920–970 nm (blaze-101) or 830–880 nm (blaze-102, blaze-112) is detected by the cameras' image sensors and interferes with the cameras' own light sources, 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 cameras 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 measurements were taken under the following conditions:
- The irradiance limit of blaze-101 cameras is 12.8 W/m², and the irradiance limit of blaze-102 and blaze-112 cameras is 6.8 W/m² (determined by Basler under specific 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 cameras' image sensors was equal to the cameras' irradiance limits of 12.8 W/m² and 6.8 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 cameras' bandpass filter (passband and stopband), and some other relevant camera properties.
The following tables list the results:
| blaze-101 | 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 |
| blaze-102 and blaze-112 | Radiometric Limit for Ambient Light Robustness (max. Irradiance @ 830–880 nm) | Photometric Limit for Ambient Light Robustness (max. Illuminance, Approximation) |
|---|---|---|
| Sunlighta | 6.8 W/m² | ~15 klux |
| Incandescent light (3200 K)b | 6.8 W/m² | ~5 klux |
| Artificial light sources without near-IR emission between 830–880 nm (e.g., visible LED, fluorescent lamp, etc.) | 6.8 W/m² | No practical limit |
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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
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Tungsten-halogen lamp, 3200 K color temperature
Effects of Ambient Light#
The effects of ambient light on the cameras' image quality are most pronounced in the depth map or point cloud. In scenes with strong ambient light in the ranges of 920–970 nm or 830–880 nm, 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 cameras' image sensors saturate. When this happens, the intensity image appears white and depth measurements are no longer possible.
Info
Ambient light isn't visible in the cameras' 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 (blaze-101) or 850 nm (blaze-102, blaze-112) LED
- 6 m distance between target and camera
- Exposure time of 250 µs
Test Setup#
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.
Test Results#
The blaze-101 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.
The blaze-102 camera still delivered usable images at an ambient irradiance of 6.8 W/m² for ambient light sources between 830–880 nm. The image noise increased by a factor of 7.