Choosing a light source for transmitted illumination

Transmitted illumination techniques such as brightfield, darkfield and phase contrast have in the past relied upon standard halogen lamps. Since LEDs for microscopy illumination arrived on the market in 2006, this has become the go-to approach and many scientists now rely on LEDs. Whether specialising in electrophysiology or spending long hours at the microscope inspecting PCBs or pathology samples, LEDs offer a wealth of benefits over traditional lamps.

CoolLED and transmitted illumination

CoolLED designs and manufactures cutting edge microscopy Illumination Systems for scientists and clinicians using the latest LED technology.

Since we introduced the first commercially available LED Illumination System in 2006, we have led the way in transforming light source technology for transmitted illumination and fluorescence microscopy. Now we are a fast-growing company in Hampshire with a vast product range and technical expertise spanning optical engineering and the life sciences.

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Benefits of LEDs for transmitted illumination

Transmitted illumination repeatability: Being solid-state, LEDs are much more stable over time, leading to a robust level of irradiance and highly repeatable experiments. Unlike halogen, colour temperate also remains constant whatever the irradiance.

Transmitted illumination user comfort: LED illumination systems lack the UV range present in halogen lamps, which is both safer and more comfortable for extended periods of working at the microscope.

Transmitted illumination control: Electronic control of LEDs is simple and fast. They can be switched on and off with precise microsecond timing and controlled remotely via TTL. Irradiance can also be finely controlled to easily achieve the optimum sample exposure, balancing sample health with image quality.

Transmitted illumination convenience: LEDs are long-lived and energy efficient, and giving off less heat reduces sample degradation.

CoolLED’s Transmitted Illumination Product Range

LED lighting systems are available for fluorescence microscopy and transmitted illumination applications.
Take a look at our product range to discover which system is best suited to your requirements.

LEDs for repeatable transmitted illumination

Being solid-state, LED illumination systems offer instant on/off and irradiance control in 1% steps, allowing the user to easily optimise sample exposure. This is one of the key advantages when comparing LEDs with the traditional 100 W halogen lamp.

Another key difference between LEDs and halogen is that colour temperature remains constant as LED irradiance is increased or decreased, which makes colour balancing unnecessary and improves standardisation between sessions. Samples will appear identical even though irradiance could be up to five times greater. This is particularly beneficial for pathology applications, allowing samples to be compared even under varying light exposures. Moreover, LEDs are renowned for their long life and stability over time.

LEDs for Repeatable Transmitted Illumination


Colour temperature remains stable with the CoolLED pT-100WHT Transmitted Illumination System which offers a broad white 100 W halogen replacement.

Safe and comfortable transmitted illumination

Halogen lighting emits wavelengths in the UV, where modern LED illumination systems such as the pT-100-WHT are designed to reduce unwanted light in the UV which makes it safer and more comfortable, as risk of eye damage is significantly lower.

Safe and Comfortable Transmitted Illumination


“Set up was very easy and the pT-100 Light Source was simple to operate. LEDs gave nice even illumination at 2% power. Higher powers were not necessary due to the brightness of the LED: 2% power with 5 msec or 10 msec exposure. There were no obvious signs of cell stress due to light toxicity over the duration of the 60min or 180min experiments I ran. Overall they are neat, simple to use and very efficient.” Tim Self, Head of School of Life Sciences imaging (SLIM), University of Nottingham, UK

Eliminating noise with narrow bandwidth transmitted illumination

Unlike halogen lamps, LED illumination systems enable the optimisation of image quality through the use of narrower spectrum wavelengths, and three narrow-bandwidth CoolLED pT-100 Transmitted Illumination Systems are available.

Eliminating Noise with Narrow Bandwidth Transmitted Illumination


White light LED systems using a phosphored LED can be a source of background noise when used as a transmitted illumination system, which reduces image contrast. Narrow bandwidth illumination systems remove this background source to improve image quality and enhance analysis – even in thicker samples and applications requiring infrared light.

Higher contrast images: pT-100-525
Narrow bandwidth at 525 nm is optimised for the spectral response of most scientific cameras and reduces background noise from a phosphored LED.

Revealing details in thick samples: pT-100-635
Narrow bandwidth at 635 nm provides deeper penetration, which is excellent for revealing detail in thicker samples.

Infrared illumination of thicker samples: pT-100-770
Narrow bandwidth at 770 nm provides deeper sample penetration techniques that require the use of infrared light.

Enhancing electrophysiology with remote light delivery

Directly coupling the light source to the microscope is the norm when it comes to transmitted illumination. But for visualising samples during sensitive electrophysiology experiments, this can cause added electrical interference and impair data quality.

To help get the most accurate electrophysiology data possible, CoolLED provides a selection of LED Illumination Systems with remote light delivery using a liquid light guide. This allows positioning outside the faraday cage, removing any possible noise (e.g. electrical interference) resulting from the Illumination System. In addition to standard direct fit models, liquid light guide variants are available for:

pT-100-WHT – A broad white light Illumination System and true 100 W halogen replacement.

pT-100-770 – Narrow bandwidth Illumination System at 770 nm for deeper light penetration and ideal for visualising thick tissue slices.

Interested? Try it out!

If you’re not sure whether an LED Illumination System is right for you, we can provide a free loan unit for testing, with no obligation to purchase. Please contact us at for more information.