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Influencing Factors#

This topic describes the factors that influence the ToF distance measurement.

Ambient Light#

由于 ToF 距离测量依赖于相机发出的光的反射,因此任何其他光(例如人造光源或日光)都会影响测量结果。

While the camera is able to measure the ambient light and then subtract it from the total light energy the pixels receive, there is a limit to the camera's ability to compensate for ambient light. The reason is that a pixel can only accept a certain amount of electrical charge. If this capacity is used up by ambient light, less space is available for the intended light pulse reflection and, as a consequence, the signal-to-noise ratio decreases.

To mitigate the influence of ambient light, the blaze-101 camera has an optical bandpass filter. This filter only allows light of the same spectrum as the camera's light source (940 nm) to pass through to the sensor. You should avoid any artificial light sources in the vicinity of your camera that emit light in that spectrum as this will lead to noise in the acquired images.

阳光不是什么大问题,因为它在 940 nm 光谱中不是很强。

For tips how to best plan your camera setup in order to avoid over-exposure, see Hardware Installation (blaze).

Scattered Light#

Reflections inside the camera lens or behind it, can create scattered light. Even though great care is taken during the manufacture of the blaze-101 camera, e.g., by using special non-reflective coating for internal components, these reflections can't be eliminated completely.

Scattered light is also easily generated by bright surfaces in the immediate vicinity of the light source. These surfaces don't even need to be in the direct field of view of the sensor to create problems. Just by placing the camera in the middle of a tabletop, the light from the camera may be reflected by the tabletop straight back into the lens and can thus skew the distance measurement. The following figure illustrates these effects.

Scattered Light Effects

为了对抗散射光的影响,Basler 采取了以下措施:

  • Careful selection of the lens
  • 限制光源孔径

信息

VCSEL 前面的保护玻璃必须保持清洁。

For tips how to best plan your camera setup in order to avoid the creation of scattered light, see Hardware Installation (blaze).

温度#

Temperature has a significant impact on the reliability of distance measurements. There are different aspects of this to consider:

  • 相机需要稳定的温度。
    即使整体温度在允许的温度范围内,测量准确性也会随温度的变化而波动。为了获得尽可能最佳的结果,温度应保持稳定。最佳温度为 22 °C。有关如何实现此目的的建议,请参阅提供散热 (blaze)
  • 相机需要预热时间。
    相机大约需要 20 分钟才能达到稳定的工作温度。由于稳定的温度是进行准确测量的先决条件,因此请在开始采集图像之前等待 20 分钟。
  • Observe the operating temperature range.
    At temperatures above 50 °C, reliable camera operation can't be guaranteed anymore. At housing temperatures below 0 °C, the camera won't start.
    The blaze-101 camera is equipped with an automatic temperature control mechanism. Therefore, even if you're operating the camera at temperatures outside the recommended range, the risk of damage or injuries is low.

Multiple Reflections (Multipath)#

For accurate distance measurement, only light reflected once delivers reliable data. Any light that has been reflected several times, by other objects in the camera's field of view or the environment in general, can falsify the measurement. Concave forms, like corners of a room or the inside of a coffee cup, are particularly problematic as the light pulse can bounce back and forth between the different surfaces, thus increasing the time until it is received by the sensor.

镜子和高反射表面(例如上漆的桌面)也可能导致多次反射,甚至可能使光 pulse 完全偏转。下图说明了这些影响。

Multiple Reflections Effects

For tips how best to plan your camera setup in order to avoid multiple reflections, see Hardware Installation (blaze).

Reflectivity of Target Objects#

The reflectivity of the target object also has an influence on the accuracy of the measurement. Two aspects need to be considered:

  • 反射类型取决于目标物体的表面质量
  • Target object color

Reflection Type#

Two kinds of reflection can occur:

  • Diffuse reflection
    This occurs in matte objects (e.g., wood or paper). Reflectance is uniform which means that the light pulses are reflected equally at all angles. In contrast to specular reflections described below, this kind of reflection is preferable because the intensity of the light reflected back into the sensor isn't influenced by the angle.
  • 镜面反射
    这发生在有光泽的物体(例如,抛光金属或极端情况下的镜子)和透明材料中。反射将根据反射定律发生,该定律规定某个方向的入射光将被反射到一个出射方向,入射和出射光 pulse 与物体表面的垂直线成相等角度。这意味着反射光 pulse 将遵循与相机发出的光 pulse 不同的路径,这可能会导致多重反射

另一个危险是反射光直接击中传感器,当光 pulse 击中物体表面的垂直线时会发生这种情况。这可能导致像素因来自单个光 pulse 的能量而变得饱和。

由于上述原因,很难对光泽或透明材料进行准确的距离测量。

Target Object Color#

明亮的物体会提供更好的结果,因为通常来说,它们能够反射更多的光,而对于黑暗的物体,则会吸收一定量的光,因此不会将其返回传感器。

Another aspect to consider is the distance of the object from the sensor. While bright objects generally yield more accurate results, if they are placed close to the sensor, they reflect back so much light that oversaturation occurs. Once pixels become oversaturated, they don't contribute to the distance measurement anymore. As a result, the measurement quality decreases.

您还需要注意,相机使用的是近红外光,并且很难预测物体是否能真正充分地反射近红外光。因此,以上区别只能作为一般的经验法则。

强度图像可以帮助您评估测量的准确性。照度良好的物体在强度图像中不会欠饱和或过饱和,将产生准确的测量结果。

无模糊#

由于光可以传播超出相机指定的范围,并且相机将此空间视为未定义区域,因此会产生模糊问题。结果是,在无模糊范围的末端,新的测量间隔再次从 0 m 开始。由于光 pulse 的定时模式,这在相机中是固有的。

这意味着超出范围的物体也会将光反射回相机。为了解决这个问题,相机将结果“折回”到无模糊范围内。

在其中一种标准操作模式下操作 blaze-101 相机时,无模糊范围为 0–30 m。超过 30 m,不会有太多的光反射回相机。因此,模糊不是什么大问题。

If you're using the Fast Mode, however, the non-ambiguity range is 0–1.5 m in Short Range and 0–10 m in Long Range. This means that only distances of objects measured in these ranges reflect the objects' true distance.

To better understand the problem, consider the following example:

范围外 1 m 处的物体将出现在无模糊范围内 1 m 处。换句话说,在远距离拍摄中,如果物体距离相机 11 m,则其距离计为 1 m。

The following figure illustrates this.

无模糊