Clinical Research Associates

Performance

Spectral Output

The spectrum of radiation that a curing light emits determines how efficiently it initiates polymerization. Most light cured dental resins use the photoinitiator camphorquinone (CQ) which readily absorbs radiation of 420 to 500 nanometer wavelengths, with peak sensitivity at 468 nm (visible blue light). A few resins use photoinitiators with lower wavelength sensitivities, usually in the upper 300 to lower 400 nanometer range (ultraviolet to violet). The great advantage of blue LED curing lights is that their narrow spectral output is near the peak sensitivity of CQ. Consequently, they polymerize most resins quickly & well with minimal power. However, their narrow blue spectrum limits their ability to polymerize the few resins that use other photoinitiators. To overcome this limitation, some manufacturers utilize only LEDs with lower wavelength output, achieving limited ability to initiate non-CQ resins, for example: bluephase 16i (Ivoclar Vivadent). Some manufacturers use two different LEDs to create a broad spectral output suitable for all resins, for example: G-Light (GC America).

The following graphs show the spectral output of each light evaluated.

Click on graph to enlarge
Avanté	FLASHlite 1401	Nova Pulse
bluephase 16i	G-Light	Radii Plus
Celalux	LED Blast	Translux Power Blue
The Cure 24	L.E.Demetron II

Power & Intensity

The measured power & intensity of curing lights depend on the test method & meter used. "Key concerns for technical leaders in the LED market are the high levels of uncertainty and inconsistencies that are associated with confusion about the methodology, design of measurement fixtures and accessories used in performing optical measurements".¹ Furthermore, there is variability among lights of the same brand, & even for the same light depending on its battery charge level & how long it has run before the measurement is taken. This inherent variability is even worse for LED lights than for conventional halogen lights because of the "significant production spread in their optical and electrical parameters"² meaning that it is difficult to manufacturer the LEDs consistently, & they are still too expensive to simply throw away those that don't meet a strict performance criteria.

1. Young R et al, "Reducing Uncertainty in Precision High-Brightness LED Measurements", Photonics Spectra, Dec. 2005, p 72.
2. MacLellan C, "Perspective on Measurement of LED Illumination Sources", Photonics Spectra, Sep. 2004, p 69.

The following chart shows power & intensity measurements made using CRA's current methods. Note the variability in results, especially between different meters & methods. Power & intensity should not be the only criteria when judging a curing light, instead, its performance with actual resin products should be considered.

1. 2.5 second maximum activation time makes stable readings difficult.
2. Accurate readings not allowed by geometies of power meter or light guide.
3. Non-conventional light guide, so an approximate emitting area was used for calculations.
4. Meter reading was not always consistent, sometimes reading significantly higher than previous measurements.

Polymerization of 4 Direct Composite Resins

Polymerization potential was compared by determining the speed of cure of a 2 mm layer of direct composite resin. Four resin brands with different chemistries were used. The universal (A3) color was used to represent typical performance, & the darkest, most opaque, slowest polymerizing color was used to represent the worst-case scenario. Minimum cure time was determined to the nearest 5 seconds by a series of samples with progressively longer cure times, progressing from uncured to cured. Cured was defined as a Barcol hardness at 2 mm, immediately following light exposure, equal to or greater than 90% of the hardness achieved using a conventional halogen light for the resin manufacturer's recommended cure time. Each test was repeated 3 times. Samples were made in a stainless steel mold 5 mm in diameter by 2 mm thick. The light guide or tip was positioned 3 mm from the top resin surface.

The following chart gives minimum cure times in seconds. Lights are listed from shortest to longest aggregate time. For comparison, three plasma arc lights & two halogen lights are also shown.

• LED
• Halogen
• Plasma arc

The geometry of this laboratory test is conservative. The stainless steel mold & black backing paper basically limit the exposure to just the light falling on the resin. The 3mm gap simulates the clinical gap between resin & light which usually exists when restoring a Class I or II restoration. The 90% hardness criteria reinforces the expectation that any new light is expected to polymerize the resin at least as well as conventional (halogen) technology. Despite the conservative nature of this test, caution should be exercised if using these cure times as a guide since they provide no margin of safety against such changes as: reduced light output, poor aim, different resin brand, etc.

Note: This test geometry favors lights with good collimation. Lights with the emitters positioned directly at the tip often have wide dispersion angles, & consequently slower times in this laboratory test, but still produce clinically acceptable results in reasonable exposure times.

Copyright © 2006 Clinical Research Associates^®