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Ambient Light Robustness#

This topic describes how the robustness to ambient light of the Basler blaze-101 has been determined.

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

  1. 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

  2. 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.

信息

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:

  • 平面白色目标,反射率 90%
  • 940 nm LED 照明目标
  • 目标和相机之间的距离为 6 m
  • Exposure 时间为 250 µs

测试设置#

照相机直接对准 6 m 处白色平面目标。为了模拟环境光,可以使用与相机波长相同的附加光源照亮目标。目标上的发光度增加到相机仍可提供不会过曝的可用图像。测量照度并基于相机图像确定噪声的增加率。

Ambient Light Robustness Test Setup

测试结果#

对于 920–970 nm 之间的环境光源,相机在 12.8 W/m² 的环境照度下仍可以提供可用图像。图像噪声增加了 7 倍。