Why all cats are grey at night and what this has to do with materials testing

“All cats are grey at night.” This familiar saying actually has a scientific basis. The retina of the human eye has two different types of photoreceptor that process light: rods and cones.

Rods enable vision in darkness or at dusk, in other words, under low-light conditions. They are so sensitive to light that they respond to individual particles of light (photons). Cones, by contrast, are considerably less sensitive to light. They are responsible for colour vision and vision in daylight and require more than a hundred photons to arrive simultaneously in order to respond.

The following figures give an impression of “bright” and “dark”: at midday on a cloudless summer day, illuminance – the amount of light available to the eye – measures approximately 100,000 lux. Around 3,000 lux corresponds to a cloudy winter day. Street lighting provides approximately 10 lux in darkness, while illuminance on a night with a full moon reaches up to 0.5 lux.

How bright is “bright”?

Typical illuminance levels illustrate the difference between bright and dark lighting conditions:

• approximately 100,000 lux at midday on a cloudless summer day
• around 3,000 lux on a cloudy winter day
• approximately 10 lux under street lighting at night
• up to 0.5 lux on a night with a full moon

As illuminance decreases, it becomes increasingly difficult for the cones to process sufficient light.

Why do colours disappear in darkness?

As the day draws to a close and illuminance falls, fewer photons reach the retina. This makes it increasingly difficult for the cones to perform their function.

Their performance begins to decline significantly at around 20 lux; below approximately 10 lux, they are largely unable to function. The eye then relies almost exclusively on the light-sensitive rods, which cannot distinguish colours. As a result, colours disappear and objects appear only in shades of grey. That is why not only cats but practically all objects and living things appear grey at night.

The connection with materials testing

So where does materials testing come in? This physiological effect also plays a role in non-destructive testing (NDT). Methods such as fluorescent magnetic particle testing (MT) and fluorescent penetrant testing (PT) are deliberately carried out under greatly reduced ambient lighting.

These methods use special testing media that accumulate at surface defects in components. When the component is then exposed to ultraviolet radiation, which is invisible to the human eye, the testing medium begins to fluoresce. It emits visible light that clearly highlights the defects.

To make these indications easy to detect, the surroundings must be sufficiently dark. Only then can the light-sensitive rods reliably perceive the small number of photons emitted.

Lighting in fluorescent testing methods

In fluorescent testing methods, ambient illuminance typically must not exceed 20 lux. At this level, the cones have only limited function, while the rods increasingly take over vision. However, this also has an implication for the inspector: because the usual division of work between rods and cones is restricted, eye strain may increase. Adaptation processes in the eye therefore also play an important role in these testing methods. The eye needs time to adjust when moving from bright to dark lighting conditions, an effect known as dark adaptation.

Even an everyday saying such as “All cats are grey at night” can therefore be explained not only in physiological terms: it also has a direct connection with the practice of non-destructive materials testing.