Using Different Wavelengths to Identify Drivers

More and more transport applications around the world are trying to identify drivers and passengers who are inside the vehicle. A camera trying to obtain clear, high quality images, inside the vehicle, requires significantly more light than it would in a typical ANPR/LPR application. In this article, we explore the difficulties of seeing through the windscreen, and discuss how different wavelengths of light could help to overcome the issues.

Challenges of Seeing Through a Windscreen

Modern windscreens are designed to block out certain wavelengths of light. This is mainly to protect the occupants of the vehicle from ultraviolet (UV) rays, but also in an attempt to keep the cabin cool by blocking Infra-Red (IR). However, the filters do not block all IR rays, just a proportion depending on the specification of the glass. This means IR can still be used for these applications, but a lot more light, and power, is required to achieve the detail necessary to identify a person inside.

One option to overcome this would be to use more power, but this could be expensive and uneconomical. However, what if we change the wavelength of light, moving away from the 850nm IR used for most ANPR/LPR applications?

Overcoming the Challenges

To analyse the effects of different wavelengths, we undertook a series of night-time tests. Using a high-performance camera, we tested four different wavelengths of light (all using the same power) to see how well they performed in a driver or passenger recognition scenario. The wavelengths tested were;

Standard IR (850nm)
Covert IR (940nm)
White-Light
Red (730nm)

Looking at the results in the image above, we can see that each different type of light achieves different results. Although the power of each illuminator is the same, the different wavelength of each means varying levels of light are able to pass through the windscreen in each scenario. Let’s summarise the results;

With standard IR (850nm), the image shows a fairly good level of detail inside the vehicle, helped by the fact the camera is highly sensitive to this wavelength.
Aside from the Covert IR (940nm), which delivers significantly worse images, at first glance, the other images do not appear to show a huge difference.
However, when we zoom further in (see images below), we can see that 730nm does provide a greater level of detail.

In more challenging applications, perhaps when the car is further away or moving at speed, the difference in quality is likely to be more pronounced.

Tinting is an additional challenge which affects the quality of images. It means even more light is blocked from passing through the windscreen. In the UK, 75% of light must be able to pass through the windscreen, meaning the tint level must be extremely low (although other countries may have more lenient laws). Generally however, darker tints can be used on rear windows, which makes identifying passengers through rear windows particularly difficult. Let’s look at the results of our different wavelength illuminators on a tinted rear window.

Here, the results are more obvious. We can see clearly that the 730nm wavelength provides the best results and allows identification of the person in the rear of the vehicle.

For a closer look at these images, click the files below;

We can summarise that; while image clarity can be improved by increasing the quantity of light, using a more appropriate wavelength can also impact the quality of an image. However, while achieving quality images is one thing, we also need to consider other factors such as the impact of light on the driver.

Other Considerations

In an ideal world, the quality of images produced using 730nm could be achieved using an illuminator which gives off no visible light (and therefore offer no distraction to the driver). However, in reality, the further down the spectrum we move, the more light that becomes visible to the driver.

Covert IR (940nm) gives off no visible light, but the images are poor. With standard IR (850nm) the quality of the images are good, but there is a visible red glow. Lastly, 730nm achieves excellent quality images with detail of the individual in the car, but there is a very obvious red glow.

The suitability of each wavelength may therefore be country or application specific (depending on the project requirements and preferences). At Raytec, we are continuing to test different wavelengths of light in an attempt to achieve quality images with minimal distraction to the driver. This includes wavelengths in-between those we have used in this particular test, but also even lower wavelengths (which produce different colours of light).

Product development

The deployment of driver and passenger recognition is a topic of hot debate, and the uptake of these systems is very much application and country specific. For example, capturing this level of personal detail may be affected by Data Protection Laws within the country of application.

At Raytec, we pride ourselves on adaptability, and in developing products in-line with our customers’ needs. We are committed to client-focussed R&D, and in testing to provide the optimum solution for driver and passenger identification. This is an ongoing process. In the meantime, to discuss your application with us further, feel free to call us on +44 (0) 1670 520 055, or email Raytec Global at sales@raytecled.com.