A bat-friendly color for lighting

Bats are strictly protected species. Disturbance of their permanent home, as well as the flight paths and hunting grounds essential for the habitat, leads to a violation of the Flora and Fauna Act. It is therefore mandatory for project initiators such as Rijkswaterstaat to take measures. As nocturnal mammals, bats are sensitive to light disturbance, both at their roosts, on flight routes and in their hunting grounds.

Night Animals

Light can disturb bats in several ways. Species that are relatively slow fly out late and always stay in the dark to avoid predation. Light forces them to fly out even later, giving them less time to hunt, or to leave the roost. Hunting areas are used less or not at all after lighting.

As nocturnal animals, bats have eyes that are evolutionarily adapted to low light intensity. Their eyes have compared to humans, relatively many rods (black and white sensitive and higher light sensitivity) and few cones (color sensitive and relatively lower sensitivity). High light sensitivity is more important to bats than seeing colors. Like human eyes, bat eyes have to get used to the dark. They start to see with the rods more than the cones at lower light intensities. Every time bats encounter light they lose their adaptation to darkness. Switching to light takes a fraction of a second, adjusting to dark takes much longer. This interferes with orientation and hunting effectiveness.

With their much more light-sensitive eyes, bats presumably also find glare simply unpleasant, possibly even painful. The light-sensitive species always try to avoid the light of flashlights immediately.

Amber

At the request of the Department of Public Works, physicists worked with bat specialists from the Mammalian Society to find a color spectrum that provides sufficient contrast and resolving power for human eyes, but is barely perceived by bat eyes. To do so, the sensitivity of colors and intensities of human eyes was compared to that of bat eyes. Because data on the specific sensitivity of bat eyes to different wavelengths (the so-called frequency response curves) for European species is not yet available, we worked with data from species from South America that are relatively close to European species.

The color/intensity sensitivity of bat eyes is shifted toward far blue and ultraviolet relative to that of humans. This means that frequencies/colors at the red end of the spectrum are much less well perceived by bats, if at all. Based on this, Maxibel created an LED lamp with a relatively monochromatic color spectrum that is expected to be "bat-friendly. This orange color has been christened Amber.

How sensitive eyes are to a particular lamp color can be calculated from the relative intensity of the wavelengths present in that lamp light, versus the sensitivity of the eyes to those particular wavelengths. From that data, the ratio in sensitivity between humans and bats could be calculated: the human to bat sensitivity ratio (Human/Bat response ratio). Based on that ratio, the suitability of different "lamp colors" can be compared. The higher the ratio the less disturbing the lamp color will be to bat eyes.

Field trial

In June and July, the amber lamp and other colors were tested in a field experiment on a flight route of the light-sensitive lake bat (Myotis dasycneme). This species follows a flight route in the Kuinderbos in the Northeast Polder over the at that location completely dark Kuindervaart. This means that about 100 animals fly past here every evening in June and July. A row of 5 floating stereo bat receivers was installed longitudinally and in the middle of the canal, picking up the echolocation sounds of the bats and registering on which side the bat flew by. On one side of the canal, 6 street lamps were temporarily mounted which, when lit, illuminated the canal exactly to the middle. This simple setup made it possible to test whether the animals try to avoid the color in question and apparently perceive it as disturbing.

Green light and white light prove not bat-friendly, amber is

The effects of different lamp colors were compared: dark/no lamp versus normal white lighting and amber. At the same time, the opportunity was taken to test the "green" lamp color that has recently been applied as a "nature-friendly lamp. Because the color spectrum of this green lamp is based on the spectrum sensitivity of birds and has actually shifted to the green-blue end of the spectrum, it was expected that bats would actually be sensitive to it. For amber, the human to bat sensitivity ratio is 45, for "normal" warm white light 8, and for green 5.3. In the field experiment, periods with the different colors were always interspersed with periods of darkness.

The results of the field test conducted by the Mammalian Society on behalf of the Department of Public Works show that bats significantly shift to the unlit side of the canal when using the white and green lights, while there was no difference between dark and Amber. In fact, fewer animals flew by during the use of the white and green lights. Apparently, some individuals even left the canal.

The green spectrum was developed as a color that does not attract birds to an oil rig and therefore does not disrupt bird migration. For bats, this color appears to be as disruptive as ordinary white light.

Because different animal species, such as birds, bats or insects have different color sensitivity, it will probably not be possible to create a color spectrum that is ideal for all species.

In this experimental setting, the amber LED lamp is proving itself as a bat-friendly color spectrum. Reason enough for Rijkswaterstaat, among other measures, to start using LED lamps in this color at sensitive bat locations on the A74 near Venlo in the near future in order to meet the requirements of the Flora and Fauna Act. 

download the fact sheet Bats and lighting here https://campingled.eu/wp-content/uploads/2025/02/Factsheets-maatregelen-vleermuizen-VERLICHTING-digitaal.pdf