Reference:Identifiers

POV-Ray allows you to define identifiers for later use in the scene file. An identifier must be at least one characters long. It may consist of upper and lower case letters, the digits 0 through 9 or an underscore character ("_"). The first character must be an alphabetic character. See also: Declaring Identifiers

Note: In versions before 3.6.2 identifier names were limited to 40 characters. There has been a Change removing that restriction.

All keywords (reserved words) are fully lower case. Therefore it is recommended that your identifiers contain at least one upper case character so it is sure to avoid conflict with reserved words.