There must be a reason, why they have a border arround the characters. I think it takes the electornics, because there's no room for it under the characters. Why? Because they need the room for generating the even light for each character. So, I think they use only one ultra bright SMD-LED for each character, with a viewing angle of 120°, put a light diffusing panel right on the top of LED and need the rest thickness of clock (20mm in total) as "diffusing path" for the light to get a well diffused light of even intensity. Another diffusing panel is used to fill the milling groove of characters and to further diffuse the light.

I don't think, that they can keep unlighted characters in the neighbourhood entirely dark, without putting all the LEDs into light chambers, with walls at the side to the neighboured characters.

A power dissipation of clock of 2W in total means a power dissipation of about 100mW per LED, then. This results in a current per LED of about 30mA for a white LED. Voltage drop at the LED is about 3.5V. A primitve constant current source could be formed by a 5V driven 74HCMOS gate in combination with a current limiting resistor. This would waste some power, of course, but would be very simple. Matched intensity LEDs are on the market, which could be further selected for equal voltage drops. Or they use drivers with built-in constant current sources at the outputs, like the ones from Allegro.

To achieve ambient light control, they could modulate all these "constant current sources" by a PWM technique. This would be extremely simple and efficient.

Kai Klaas