This link gives information regarding the visibility of numbers. Numbers below 4" or 10 cm in height are too small for reliable visualization from longer distances especially at night. Added to this problem are the issues of contrast, visualization during day and night time hours. Solar-ID numbers are white on black contrast. The number is outlined with 8-12 brght LEDs giving superior night time visibility compared to back lit outlined numbers or indirect lighting of numbers or letters at night.
Solar-ID numbers are 4" tall and each number has 8-12 bright, warm LEDs (1800 milli candelas each) for illumination at night. That's about equivalent to a 15 watt bulb. When they turn on automatically at night, the numbers give the impression you are at home and are prepared to protect your security and your safety. You don't have to remember to turn the Solar-ID numbers on at night time. There aren't any complicated wires to install and your are using the power of the sun to provide the energy.