Remote phosphors have been used in a number of competing LED lighting products, and are known as one route to increase fixture efficiency. Removing the heat generating phosphor from the LED chip and allowing it to be independently dissipated through the lens or case assembly enables the LED junction to maintain a lower temperature for the same luminous output. Prototype films have been made in-house at CRS Electronics to assess coating of phosphor powders to generate custom emission spectra for individual projects or special applications.
Principles of Operation
All single die white LEDs currently on the market make use of phosphors to achieve white light from the narrow band emitted from the semiconductor junction. While generally known as phosphors, the active materials in are actually fluorescent materials. As used in LED applications, incident light (typically Blue for white LED chips or fixtures) is absorbed by the phosphor layer, which then fluoresces, emitting light at a longer wavelength than the light initially absorbed. The combination of the longer wavelength light emitted from the phosphor and transmitted shorter wavelength light from the LED die combine to give a white appearance to the resulting spectra.
When a phosphor atom or molecule absorbs a photon from the LED die, an electron is excited from its ground state in the phosphor to a higher energy state. From this initial excited state the molecule loses energy (producing heat) as it passes through one or more intermediate states until it drops into the lowest vibrational level of the excited state. From this state the molecule undergoes a radiative transition back into the ground state, emitting a photon of a longer. The absorption to emission cycle typically takes 0.5 ns to 20 ns, and only once the process is complete can the same atom or molecule repeat the process.
The energy loss between the time that a photon is absorbed and when a photon is emitted is known as the Stokes shift. For example, if a phosphor absorbs a blue photon at 450 nm and emits a red photon at 650 nm the energy efficiency of the process is 69%, with 31% of the initial energy being converted to heat in the phosphor layer. The absolute efficiency of a phosphor system is equal to the multiplication of the quantum yield of the material (the ratio of emitted photons to absorbed photons) and the ratio of the emitted photon energy to the energy of the absorbed photon. Typical white-light remote phosphor conversion efficiencies are on the order of 180lm/Wrad (180 lumens out, given 1 Watt radiometric power of Royal Blue light incident on the remote phosphor layer – Source: Intematix ChromaLit documentation). As there is no relationship between the direction of the absorbed light and the emitted light, and incident light that is not absorbed has a high probability of being scattered, use of remote phosphors will always produce a wide angle emission profile.
Phosphor containing films were made by pouring the viscous phosphor-encapsulant slurry onto a flat panel with spacers placed around the edges, and then sandwiching another flat panel on top to spread out the mixture. In these prototypes a series of washers with a height of 0.8mm were used resulting in films that are approximately 0.9 mm thick. Once the slurry was sandwiched between the two plates, it was cured at 100°C for 1 hour. In early samples there were numerous air bubbles. With better de-gassing technique and a longer waiting period between slurry pour and cover placement films were made with few bubbles and very good uniformity. Using this method films can be made at least 14” by 14”, though if used for production, spray coating or subcontracting the coating to Intematix or other companies may be cost effective.
A spectrometer test test jig was constructed out of a cardboard box, with a cut-out at on end for the Ocean Optics spectrometer. Two interchangeable panels were made for the opposite end to fit the Royal Blue luminaire fixtures as test platforms (see Figure 1 for the test jig with luminaire installed).