Night Vision and Day Vision - the Differences

Blindingly bright days and pitch-black nights: human eyes can adapt to enormous differences in brightness, although the retinal cells responsible for night vision cannot distinguish colours. In fact, photopic vision (day vision), scotopic vision (night vision) and mesopic vision (in twilight) function differently.

The difference between photopic, mesopic and scotopic vision

The human eye perceives brightness and colours in different ways:

• Photopic vision (day vision)

Daylight is detected by the so-called cones in the retina, of which there are three different types for the primary colours blue, red and green. They are specialised for bright light and enable sharp colour vision as well as the perception of fine details.

• Scotopic vision (night vision)

Night vision is handled by the sensitive rods, which can detect even very faint light stimuli in the dark, but can only distinguish between levels of brightness and shades of grey. Colours remain invisible, and contrasts also appear significantly weaker than during the day. Visual acuity decreases significantly during night vision, so that even in relatively good lighting conditions (e.g. on a clear full-moon night), details are harder to make out.

• Mesopic vision (twilight vision)

Between these two states lies mesopic vision at twilight, in which both cones and rods are active.

From daytime vision to night vision

The eye requires around 25 minutes to switch between daytime and night vision (dark adaptation). During this process, it must adapt to extremely different luminance levels. As it gets darker, the pupil dilates up to 16 times its diameter at maximum brightness. This allows more light rays to reach the retina. At twilight, both cones and rods are involved in vision.

The V-Lambda curve

The V-Lambda curve

The so-called V-Lambda curve illustrates that other wavelengths are perceived as the brightest during daytime and night-time vision (luminance sensitivity). For example, blue tones appear brighter during twilight and at night than in midday sunlight, an effect that is also utilised in cinematic artificial lighting (‘American night’). Once dark adaptation has taken place – that is, once the eyes have adjusted and become accustomed to the new visual conditions under a dark sky – one can usually find one’s way even in faint starlight.

The right lighting for the eyes when switching from day to night vision

When twilight sets in, our eyes must first adapt to the changed lighting conditions – this process is called dark adaptation. To ensure this transition is as comfortable as possible for the eyes, the right lighting plays a crucial role.

• Creating gentle transitions: Dimmable lamps or smart lighting systems allow the brightness to be reduced step by step. This allows the eyes to get used to the darkness without sudden strain.
• Avoid glare: Excessive contrasts between light and dark areas strain the eyes unnecessarily. Warm white light sources (up to approx. 3,300 Kelvin) with indirect illumination are ideal here.
  
  
• Adjust the colour temperature: Warm light in the evening signals to the body that it is time to wind down – much like natural light at sunset.

The right lighting therefore not only supports the visual process but also contributes to greater safety and well-being.