From FAA Flight Inspection Manual:
SECTION 1. VISUAL GLIDE SLOPE INDICATOR (VGSI)
7.10 INTRODUCTION. The Visual Glide Slope Indicators (VGSI) are ground devices that use lights to define a vertical approach path during the final approach to a runway. The visual signal must consist of not less than two and not more than four colors. Allowable colors are red, amber, green, or white. Color sectors must be distinct and identifiable throughout the horizontal beam width at all intensity settings. Only red is used to indicate the lowest below-path sector of the system.
The final approach area for VGSI(s) is 10° either side of the runway centerline extended, measured from the forward most bar or light extending from the threshold outward to a point a normal glidepath can commence from the en route or procedural altitude. VGSI(s) are aligned to provide a glidepath not less than 1.0° above obstacles 10° either side of the runway centerline to a distance specified for the system, usually 4 miles. Lateral guidance is obtained by reference to either visual cues or electronic aids.
Threshold crossing height (TCH) is the height of the lowest on-path signal at the threshold. The minimum TCH is determined by the most critical aircraft that normally operates on the runway. The TCH of VGSI(s) will normally be 25 to 75 ft. Specific TCH criteria for each type system is located in FAA Order 6850.2, Visual Guidance Lighting Systems.
Box Identification. The U.S. practice, as found in FAA Order 6850.5, Maintenance of Lighted Navigational Aids, is that individual VASI or PAPI light boxes are numbered starting at (1), with the box nearest the runway on each side and working outboard. ICAO Annex 14 and Aerodrome Design Manual reverse this, and number or letter the boxes starting with (1) or (A) at the outermost box and working toward the runway.
There are several different types of VGSI(s). The primary systems covered in this chapter are visual approach slope indicators (VASI), precision approach path indicators (PAPI), pulsating visual approach slope indicators (PVASI), T-VASI, three-color VASI, and helicopter approach path indicator (HAPI). Each of these systems presents a different type of visual indication to the pilot and requires different in-flight interpretation.