Optitron meters are constantly backlit with higher intensity of light even during the day unlike and ordinary meter which only lights up when lights are switched on with normal less bright bulbs and stay non backlit during the day.
In a normal meter even if you switch the lights on during daytime the lights won’t be visible in meter. In simple words optitron meters glow constantly.
To further clarify difference, check local GLi which has a regular meter while Altis has a backlit glowing optitron meter