Usually, seven-segment displays emit red coloured light, which is understandable. But in digital watches, the common colour used for the numbers displayed is black.

picture of seven segment watch diaplay

But black light itself can't be emitted. So the display must be absorbing light in some way instead, but I'm not able to find any resource describing the exact working of these displays.

One interesting thing I have noticed is that when I view the watch at an angle, I can view all the outlines of the seven-segment display (though it has a lot more than seven segments). But when I checked another watch that was out of charge, it didn't default all segments to black nor did it have any outlines at any angle.

powered watch showing outlines of all segments at an angle

unpowered watch not showing any outlines of segments

I believe this means that there are some mechanisms actively working to produce this black colour. I can rule out the possibility that the inactive segments are made to give out light in the colour of their background.

I also don't think the answer is as simple as the entire background glowing extra bright, because that would be inefficient, and it doesn't change its intensity so much that we can notice it, yet the segments are very black when displaying a number.

  • 1
    \$\begingroup\$ ...entire background glowing extra bright, because that would be inefficient..." This display does depend on ambient light as its light source. Using available light is very efficient, since it is not powered by the watch battery. Background surface (behind the LCD) is an efficient reflector and diffuser of available light coming in. \$\endgroup\$
    – glen_geek
    May 27, 2022 at 12:37

2 Answers 2


7 segment means the layout of the display.

It can be implemented in various ways, such as red LEDs, or with liquid crystals, or flipping coloured plastic parts with electromagnets.

The display you are asking about is a liquid crystal display, or an LCD.

It has a chemical between electrodes that can polarize light, and a polarization filter at top. So when the segment is not powered, it will be transparent as it allows light to pass and reflect back. When the segment is powered, it will only pass light that is polarized correctly and together with the filter it appears that it blocks light from reflecting back.

So now you can search for further info how liquid crystal displays work.

  • \$\begingroup\$ "it blocks light from reflecting back" doesn't it block from passing through in the first place? \$\endgroup\$
    – Balu
    May 27, 2022 at 17:23

The usual red (or coloured) seven segment displays use Light Emitting Diodes (LEDs).

The displays on watches are Liquid Crystal Displays (LCD) - a totally different technology.

  • \$\begingroup\$ Yes, I'm aware, that was not my doubt itself, it was just an intro to what I was talking about \$\endgroup\$
    – Balu
    May 27, 2022 at 16:41

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