Lighting Comparison: LED Versus HID Lights

- Aug 06, 2019-

High Intensity Discharge is an overarching term for a gas-discharge light. They are the oldest type of electrical light  Among the common types of HID lights are mercury vapor, low and high pressure sodium and metal halide lamps. Other less common variants include ceramic metal halide and xenon short-arc lamps. HID lamps produce light by sending an electrical charge or “arc” between two tungsten electrical conductors (electrodes) and through an ionized gas (also known as “plasma”) which is housed inside the bulb. HID lights require ignition which is typically provided by a voltage pulse or a third electrode (an additional metal part) internal to the bulb. Once lit the electrical arc begins to evaporate the metal salts inside the bulb which significantly increases the luminous power of the bulb while simultaneously improving lighting efficiency. HID lighting requires a “warm-up” period because the lighting intensity is dependent on and changes as the material inside the bulb is evaporated into plasma. Additionally, as the light heats up it requires additional voltage to operate. Voltage requirements in HID bulbs are balanced by an electrical ballast (essentially a device that limits electrical current to that required to operate the lamp). As the HID light ages, more and more voltage is required to produce the same amount of light until eventually the voltage exceeds the fixed resistance provided by the ballast and the light goes out (fails). HID lights become less and less efficient over time because they must use more and more voltage to produce the same lumen output as the light degrades.

HID technology has been around for several centuries and is typically used when high intensity, high efficiency, or lighting over a vast area is required. New HID lamps produce more visible light per unit of energy than both incandescent and fluorescent lamps because a smaller proportion of the energy emitted is in the infrared spectrum (i.e. more is in the visible light spectrum). Generally speaking they are efficient and produce a high quality light.

Amongst the deficiencies in HID lighting are the following:

  1. A portion (roughly 30%) of the energy emitted by HID lights is infrared (which in terms of lighting output means it’s entirely wasted energy). Although this figure is worse for older variants of the technology and better for new HID bulbs, it’s a relevant inefficiency in either case. Of note, both incandescent and fluorescent bulbs are worse than HID with respect to the percentage of radiation that is infrared vice visible light.

  2. HID lumen output can significantly deteriorate as the bulb ages. Some HID bulbs produce 70% less visible light after only 10,000 hours of operation.  

  3. Most HID lighting emits a significant amount of UV radiation. Due to this deficiency HID lamps require UV filters to prevent fading of dyed items exposed to their light, degradation of lamp fixture parts, or serious injury (sunburn or arc eye) to humans and animals.

  4. HID lights are omnidirectional. Omnidirectional lights produce light in 360 degrees. This is a large system inefficiency because at least half of the light needs to be reflected and redirected to the desired area being illuminated. The need for reflection and redirection of light means that the output is much less efficient for omnidirectional lights due to losses than it would be for the same light if it were directional by its nature. It also means that more accessory parts are required in the light fixture itself in order to reflect or focus the luminous output of the bulb (thus increasing unit costs).