Thermal imaging & vision systems


should not affect the accuracy, the choice of wavelength can affect the system-level performance in some use cases. The following are some considerations when choosing the wavelength.


Sensor quantum efficiency and responsivity:


Quantum efficiency (QE) and responsivity (R) are linked to each other.


QE measures the ability of a photodetector to convert photons into electrons.


Case 1 2 3 4 5


R measures the ability of a photodetector to convert optical power into electric current


Human perception


While the human eye is insensitive in the near infrared (NIR) wavelength range, light at 850 nm can be perceived by the human eye. On the other hand, 940 nm is invisible to the human eye.


where q is electron charge, h is plank constant, c is speed of light, and is wavelength.


Typically, the QE of silicon-based sensors is about 2× better or more at 850 nm than at 940 nm. For example, ADI CW ToF sensors have 44% QE at 850 nm and 27% QE at 940 nm. For the same amount of illumination optical power, higher QE and R lead to better signal-to-noise ratio (SNR), especially when not much light returns to the sensor (which is the case for faraway or low reflectivity objects).


Sunlight


Although the solar emission is maximum in the visible region of the spectrum, the energy in the NIR region is still significant. Sunlight (and ambient light more generally) can increase depth noise and reduce the range of a ToF camera. Fortunately, due to atmospheric absorption, there is a dip in sunlight irradiance in the 920 nm to 960 nm region, where the solar irradiance is less than half compared to the 850 nm region (see Figure 3). In outdoor applications, operating the ToF system at 940 nm provides better ambient


Horizontal FOI 60° 52° 60° 72° 78° Vertical FOI 45° 52° 60° 58° 65° Table 1. Normalised Radiant Intensity


light immunity and leads to better depth sensing performance.


RADIANT INTENSITY (OPTICAL POWER PER SOLID ANGLE)


The light source generates a constant optical power distributed into a 3D space within the FOI produced by the diffusing optics. As the FOI increases, the energy sustained per steradian (sr)—that is, radiant intensity [W/sr]—decreases. It is important to understand the trade-offs between FOI and radiant intensity as they affect the SNR, and therefore the depth range, of the ToF system. Table 1 lists a few examples of FOI and their corresponding radiant intensity normalised to the radiant intensity of a 60° × 45° FOI. Note that the radiant intensity is calculated as optical power per rectangular solid angle.


ILLUMINATION PROFILE SPECIFICATIONS


To fully define the illumination profile, several characteristics should be clearly specified including the profile shape, profile width, optical efficiency (that is, enclosed energy within a certain FOV), and optical power drop-off outside the FOI. The illumination profile specification is normally defined in radiant intensity in angular space. Mathematically it is expressed as:


Normalized Radiant Intensity


100% 100% 76% 67% 56%


where d is the power emitted into the solid angle dΩ. The FOI needs to match the aspect ratio of the imager, and hence is normally square or rectangular.


Figure 3. Solar spectral irradiance in NIR. Instrumentation Monthly April 2025


Illumination profile shape inside FOI The most common radiant intensity profiles in ToF flood illumination have a batwing shape. They have a profile that varies in cos-n ( ) to compensate for the drop-off (that is, relative illumination) of the imaging lens. Figure 5


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