In the 1930s, it has been experienced that certain dyes and resins combinations produce brighter colours than normal colours and had the unique effect of “glowing” under ultraviolet or black light.
Fluorescent coloured objects are seen three times earlier than objects coloured with Conventional Pigments due to the fluorescent light given off, meaning they are utilized in applications where catching the eye is beneficial. As the chemistry and manufacturing process improved, the areas of application expanded to advertising. Safety and promotional firms began to recognize the uniqueness of these bright colours and their special uses.
DAYLIGHT FLUORESCENT COLOURS :
Daylight Fluorescent Pigments can create vivid and eye-catching effects as compared to Conventional Daylight Pigments.
For example, the Fluorescent orange colour absorbs the same orange band as the conventional, however, it also converts the lower end of the spectrum and ultraviolet light into visible light of the predominant wavelength. Conventional/normal colour can reflect only light in the visible range, in the case of Fluorescent Colours, it even converts absorbed UV light and reflects in the visible range, thereby colour appears brighter than normal colour.
When a clean, bright conventional colour is able to reflect a maximum of 90% of the colour present in the spectrum, a Fluorescent Colour can reflect 200-300% of the colour present in the spectrum.
MECHANISM OF DAYLIGHT FLUORESCENT COLOURS :
Daylight Fluorescent Colours occur when higher energy, short-wavelength light is absorbed and instantaneously emitted at wavelengths longer than those of absorption. The addition of the reflected light components can bring the brightness up to three times that of conventional pigments were a lot of the energy is merely absorbed and dissipated as heat.
Fluorescent colours are generally more fugitive than similar hue conventional colours; the relative degree of light stability would depend on formulations, film deposits, wall thickness, additives, etc., items as well as the areas of the intended use.
LIMITATIONS OF DAYLIGHT FLUORESCENT COLOURS :
Daylight Fluorescent Pigments are stable to indoor light or outdoor conditions other than direct sunlight as they are affected by exposure to direct sunlight. The degree and effect of the change are dependent on the colour, intended end usage, pigment loading, and other important factors including :
- Type of vehicle/binder and overcoat.
- Type of plastic (Ex: Vinyl will normally give significantly better results than polyolefin).
- The thickness of the pigmented coatings or wall thickness of the plastic.
- The pigment concentration/loading. (Higher the loading, better the light fastness – in general).
If prolonged outdoor exposure is intended use for the fluorescent-coloured item, actual outdoor exposure tests should be conducted to be certain of satisfactory results. Accelerated testing, such as carbon arc/ xenon weather meter, will give comparative indications of light fastness, but there is no exact correlation between accelerated and actual outdoor weathering exposure.
Lightfastness can be improved to an extent by :
- Higher pigmentation level.
- Ultraviolet absorbers in the pigmented formulations.
- Clear acrylic overcoats (or) clear overcoats with UV absorbers.
All these systems should also be tested to assure satisfactory results.