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Electron Microscopy Sciences

Technical Data Sheets

Flashing Excitation for Enhanced Detection of Fluorescence in Ambient Light

The Xite flashlights include a flashing mode that can enhance the ability to detect fluorescence in the presence of ambient light. The more contrast there is between a subject and its background, the easier it will be to detect. While fluorescence may appear bright when viewed in conditions of darkness, in actuality it tends to be a relatively weak effect that is easily masked by other light. If you pass a fluorescence excitation source over a scene that is illuminated by ambient light (room light, sunlight, etc.), any fluorescing subject will respond to that excitation, but the response may be too weak to notice easily. The ability to detect these small responses can be enhanced by making them flash repetitively, a kind of strobe effect. This can be done by making the excitation source flash. The fluorescence response is for all practical purposes instantaneous, so it will flash at the same rate as the excitation source. This flickering effect increases the apparent contrast and thus increases detectability.
The benefit of flashing for detection is related to two factors of human visual perception:

  1. Humans are naturally drawn to changing stimuli.
  2. A light flashing with appropriate characteristics is perceived to be brighter than a steady light of the same intensity.
The first factor is something we know from our daily lives. When you want to attract someone's attention you don't stand like a statue, you wave our arms. Warning lights designed to attract our attention (railroad crossings, police lights) don't generally move in space, but create the same effect by flashing.
The second factor is known as the Brücke-Bartley effect, and it has been demonstrated experimentally many times. While a blinking (changing) stimulus naturally draws attention (point 1), this visual phenomenon (point 2) further increases the contrast of that stimulus.
In order to exploit these effects we have incorporated a flashing mode in the Xite flashlight. When used in combination with barrier filter glasses matched to the source, the operator can see any flashing fluorescence but will not see the flashing of the light source itself. Any non-fluorescing subjects will appear normal, although with a color tint associated with the filter glasses. As the light source is moved through the scene in a search mode there will be an increase, often small, in brightness of any fluorescing subject when the excitation light falls on it. With the light source flashing, that increased brightness will flash at the same rate, enhancing our ability to see it.
This approach will not work in all conditions. If the ambient light is just too bright the small flashing signal will still not be detectable. If you want to search defined surfaces in very bright conditions you can use an opaque object to cast a shadow on the area that you are viewing. This technique was used successfully in searching for baby corals (on the order of 1mm diameter) underwater on natural reef surfaces in shallow water in the middle of the day, using a clipboard to shade the surface.
We do not recommend using this mode when you actually are in the dark or low light. In these cases the flashing effect has no real benefit over steady illumination and is generally annoying- it's like trying to work in a disco with a strobe light and it is very disorienting.
Safety note: Some people are very susceptible to flashing lights, which can trigger seizures (photosensitive epilepsy). Do not use this mode if you or anyone in the vicinity has this sensitivity. The Xite light is programmed so that you have to use an intentional rapid-press sequence of the mode button to activate it.