Camera sensors for military use: sensor technologies

The first article in this series, Camera Sensors for Military Use: Design Fundamentals, covered the main variables to consider: lens, functionality and SWaP (size, weight and power), as well as performance parameters and trade-offs. In this article, we explore CCD, EMCCD, and CMOS. These are all silicon based detectors, as such they are generally sensitive in the UV, visible and near infrared regions of the spectrum. SWIR cameras are InGaAs focal plane arrays which, as the name suggests, are sensitive in the shortwave infrared region of the spectrum. In addition, each sensor technology has its own peculiarities:

• On a CCD, all pixels are read through an A / D output and read noise increases with frame rate but can be reduced by binning. Sensitivity, speed and spatial resolution are related and subject to tradeoffs and tradeoffs.

• On an EMCCD, the read noise can be compensated by the gain of electron multiplication before the A / D output. This makes EMCCD the most sensitive silicon-based device and removes the link between sensitivity, speed and spatial resolution. Unfortunately, very few EMCCD chips are available on the market.

• With a CMOS, all pixels can ben be read simultaneously since each has its own A / D converter. It also removes the link between sensitivity, speed and spatial resolution. However, the A / D of each pixel has slightly different offset, gain, and dark current characteristics that require non-uniformity correction to emulate the image quality of a single output device. In addition, binning will not reduce reading noise.

Table 1: The characteristics of cameras using these different sensor technologies

Irradiance, a standardized value

With so many settings, it can be difficult to estimate which camera to select in order to get the best possible performance for a given application. A few basic questions regarding the spectral range, spatial and temporal resolution required can already help narrow the choice. For example, the choice of a camera suitable for hyperspectral imaging is therefore often mainly guided by these three parameters, QE, pixel size and exposure time. However, it would be useful to reduce all of these parameters to one.

Noise equivalent power (NEP) is the minimum power that can be detected, measured in watts. It does not take into account the surface of the pixels. It is typically used for photodetectors and standardized for an output bandwidth of one hertz or an integration time of half a second. As such, it is not a practical setting for comparing cameras with very different pixel sizes and covering a range or exposure time. The lower the NEP, the higher the sensitivity.

Specific Detective (D *): is the inverse of the NEP normalized on the square root of the photosensitive zone. It is expressed in cm√Hz / W or Jones, unlike NEP, the higher the specific detectivity, the higher the sensitivity. However, it still does not reflect the wide range of possible exposure times.

Irradiance is the power of electromagnetic radiation per unit area (radiative flux) incident on a surface its SI unit is watt per square meter (W / m²). Noise Equivalent Irradiance (NEI) is the luminous flux density required to equal camera noise. NEI is usually expressed in W / cm² or in Photons / (cm²s).

The advantage of NEI is that it offers a single standardized quantity representing the sensitivity which can be calculated from the specifications provided by the manufacturer. The lower the NEI, the higher the sensitivity.

The general noise equation is as follows:

Since only reading noise and dark current are dependent on the camera. We can define the NEI at a given wavelength as follows:

Numerical example

For a 640×512 Vis-SWIR camera in High Gain mode at 1550 nm, typical values ​​are as follows:

• QE is 80% at 1550 nm

• The pixel size is 15×15µm

• Full well capacity, 12,000 th^–

• reading noise is 50th^–/ RMS pixel;

• dark current is 2.5 fA / pixel (at 15 ° C sensor temperature)

Remembering that:

Using an exposure time of 33 ms, the NEI at 1550 nm will be:

NEI to W / cm conversion²:

To obtain the irradiance equivalent to noise in W / cm², it is necessary to use the energy per photon, which is determined by the wavelength according to the following relation:

• Plate constant h = 6.626 069 573 × 10^−34 J ∙ s

• Speed ​​of light c = 299 792 458 m / s

The third part of this series on the choice of cameras for military applications details the noise equivalent irradiation (NEI) for each type of sensor.

Written by Jean-Edouard Communal, Sales Director at Raptor Photonics Ltd.

Read the whole Military Use Camera Sensor Series here:

Camera sensors for military use: basic design principles

Camera sensors for military use: sensor technologies

Camera sensors for military use: sensitivity comparison