Light Sources

 

Pulsed Lightsources for Time-Gated Imaging

Which light sources are most suitable for Time-Gated and  Fluorescence Lifetime Imaging?
The answer to this question depends on many factors such as the fluorescence lifetime and spectral excitation ranges of interest, budget, required stability levels, size and weight considerations etc. A brief and necessarily incomplete list of commonly used pulsed lightsources is shown below:

 

Light Source Type

Recommended Lifetime Range

Wavelength Range

Guideline Cost

Custom/Commercial Pulsed LED System

>250nsec

UV/Visible/IR

>2000

Nitrogen Laser

>100nsec

337.5nm

c.4000

DPSS Laser

1-200nsec

266/355/532nm

>12,000

‘Home-made’ Pulsed LED

>100nsec

UV/Visible/IR

c.100-1000

Pulsed Xenon Flashlamp

>10usec

UV/Visible/IR

>4,000

 

Custom/Commercial Pulsed LED Systems
LEDs offer a Low Cost and very reliable solution for many time-resolved measurements. LEDs are available in a variety of wavelength outputs, but we have most experience with the UV (250-400 nm), blue (470nm peak) and green diodes (530nm peak). A 'white' LED is also available, where a phosphor coating is used for wavelength conversion. Some of diodes can be pulsed in nanoseconds or less and modulated at frequencies up to c.1-200MHz. Care must be exercised as some diodes have substantial batch-to-batch variations and not all 'colours' are equally suitable for very high speed use. The minimum pulse width of the phosphor-coated white diode is of the order of 100ns in our experience based on very few samples. This relatively slow response is probably limited by the phosphor coating. The LEDs are extremely stable in operation and offer high intrinsic brightness because of the small source size. Output power levels can range into the 1000s of mWatts.
 

Nitrogen Lasers
Nitrogen lasers are also very suitable for time-resolved measurements requiring a UV excitation. Typical pulse widths are between 300ps and 10ns and repetition rates are usually up to c. 20Hz for low cost commercial systems. Basic sealed module Nitrogen lasers are compact and reliable and give very high peak power levels (hundreds of kilowatts or more) at average power levels of a few milliwatts for a small laser. Cost is typically a few thousand pounds for a sealed unit that requires no external nitrogen source. The output is at 337nm in the UV, which can excite most fluorescent materials, but gives a lot of background from optical filters, lenses etc. Addition of a dye module gives access to a wide range of wavelengths across the visible spectrum.
 

DPSS lasers
Diode-Pumped Solid State lasers combine high power infrared emitters with a non-linear optical frequency doubling or tripling stage to give collimated visible or UV emission. When combined with an active Q-Switch such lasers can make excellent Pulsed light sources and we have used these successfully with the Imagex-nanoCCD system for nanosecond lifetime Imaging down to the sub-nanosecond scale. We can recommend the ‘SPOT’ series DPSS laser from Elforlight Ltd, UK (www.elforlight.com)
 

Xenon flashlamps
For long lifetime samples (>20 microseconds or so) a fast pulsed Xenon flashlamp is a good compact light source. This gives output from UV to IR and is intense, compact and relatively inexpensive. Some flashlamps suffer from significant 'afterglow', particularly at longer wavelengths and this aspect needs to be considered for each application. Xenon flashlamps are typically pulsed at several hundred Hz with microsecond pulse widths. Faster flashlamps with sub-microsecond pulse widths are available commercially for special applications. We no longer supply XEnon Flashlamp systems due to EU regulations however it is possible to buy yhe major components from XXX to construct a relatively simple user-configured system. Alternatively complete compact Flashlamp systems can be purchased from Hamamatsu (www.hamamatsu.com)

PRS have considerable experience of the use of such diodes for excitation of fluorescence and in our opinion this source is the first choice for pulsed and modulated operation if the emission wavelength is suitable for the label of interest.