According to the Illuminating Engineering Society Lighting Handbook, the CRI measures how a lamp's light compares with "a reference source of the same chromacity." This means that sometimes the reference point is sunlight and sometimes it's not. For very bright bulbs, the handbook explains, the reference source is an agreed-upon form of daylight; for dimmer bulbs it's a block of heated iron. The handbook then goes on to point out various inconsistencies in the system, but concludes: "Why use the CRI if it has so many drawbacks? It's the only internationally agreed upon color rendering system that does provide some guidance."