Non-phosphor white LED light source for interferometry

Abstract

Solid state light sources are replacing a tungsten filament based bulbs in Scanning White Light Interferometers. White LEDs generate little heat, feature short switching times, and have long lifetimes. Phosphor-based white LEDs produce a wide spectrum but have two separate peaks which cause interferogram ringing. This makes measuring multi layered structures difficult and may degrade measurement precision even when measuring a single reflecting surface. Most non phosphor white LEDs exhibit a non Gaussian spectrum, but multi-LED based white LEDs can achieve switching times and stability similar to those of single color LEDs. By combining several LEDs and by controlling their input current independently it is possible to create almost an arbitrary spectrum.We designed a new light source by combining a non phosphor white LED (American Opto Plus LED, L-513NPWC-15D) and single color LEDs. This allowed us to fill the spectral gap between the blue and yellow peaks of the non phosphor white LED. By controlling the input current of the LEDs individually a nearly Gaussian shaped spectrum was achieved. This wide continuous spectrum creates short interferograms (FWHM ~1.4 µm) without side peaks. To demonstrate the properties of this source we measured through a 5 µm thick polymer film. The well localized interference created by the source allows measuring both surfaces of thin films simultaneously. We were able to pulse the source at 5.4 MHz.

Harnessing spectral property of dual wavelength white LED to improve vertical scanning interferometry

“…Other than the designed and the undesired spectral variations mentioned above, the intensity spectrum of the dual wavelength white LED can vary depending on its operating condition [5,15] and optical degradation [16,17]. For example, (1) when the white LED is being pulsed at high frequency, the relative intensity of the blue and yellow light components changes [5], and (2) the center wavelength of blue light shifts by 7 nm when the duty cycle in pulsing mode is changed from 1% to 10% [15].…”

“…For example, (1) when the white LED is being pulsed at high frequency, the relative intensity of the blue and yellow light components changes [5], and (2) the center wavelength of blue light shifts by 7 nm when the duty cycle in pulsing mode is changed from 1% to 10% [15].…”

“…Researchers [4,5] have reported that the use of the dual wavelength white LED introduces a distinctive feature, which means that the fringe contrast function cannot be modeled as a single Gaussian function, and the distinctive feature may affect the measurement result negatively. Although it has been shown that the distinctive feature is the result of destructive interference between the blue and yellow lights of the dual wavelength white LED [20], the work has focused on the dual wavelength low coherence light source and has not further investigated the dual wavelength white LED in detail.…”

“…Driven by recent advancement in lighting technology and new applications such as dynamic characterization of micro-electro-mechanical systems (MEMS), modern and hybrid light sources such as supercontinuum light source [1][2][3], phosphor-based white LED [4], nonphosphor white LED [5], a combination of two low coherence light source [6], etc., have been used as the light source in vertical scanning interferometry. Among these, the high power white LED is the most commonly used solution due to its advantages in modulation frequency, brightness, affordability, and availability.…”

Spectral effects of phosphor-based white light-emitting-diode (LED) in coherence scanning interferometry

“…[88] and is expected to last longer [89], [90]. Other than a achieving high modulation frequency, Heikkinen et al [61] also found that there is spectral variation at different pulsing frequency. As shown in Figure 2-22, it is found that the ratio between the blue and yellow lights of the non phosphor-based white LED changes with pulsing frequency and the wavelength of the yellow light also increases as the pulsing frequency increases.…”