martes, 3 de mayo de 2011

Controlling LEDs Part 6 - What is the dimming range of the product?

Incandescent lamps dim to below one percent perceived light, which looks like an orange filament glow. The dimming range of an LED lamp or fixture can vary greatly from one device to another.
Some may dim to a minimum level of only 50 percent, while a different product may dim to one percent. Additionally, manufacturers will quote measured light numbers, but consumers are familiar with perceived light. 

What is the difference?

Measured light output is the quantifiable value of light measured by a light meter or similar device. This is the dimming percentage indicated on LED product specification sheets.

Perceived light is the amount of light that your eye interprets because of pupil dilation. The eye’s pupil dilates at lower light levels, causing the amount of light to be perceived higher than measured (e.g., 20 percent measured light equals 45 percent perceived light). The equation for determining perceived light is to take the square root of the measured light percentage (e.g. √0.2 = 0.45). You need to select the dimming range of your fixture or lamp that will be suitable for your application. A product that dims to 20 percent measured light (45 percent perceived) wouldn’t make sense in a media room, but may be the energy-saving solution necessary for an office. If an LED fixture or lamp spec sheet does not state the dimming range, you should contact the manufacturer for that important piece of information.

It is important to know that the dimming range of a product is based solely on the driver. The integral driver will determine the dimmability of a screw-in LED retrofit lamp and an external driver will determine the dimmability of an LED fixture. Each dimmer may have varying features that will affect your ability to trim out flicker if it exists and minimum load requirements, but it will not affect the dimmability of a product. The driver will determine the low-end light level and the performance. Finally, be aware that the dimming  range of a single product may vary based on what control is used.