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does not change during dimming. So, the incandescent bulb gives you both an intensity and color
temperature variation, while LED only provides an intensity variation and the color temperature will
remain the same.
People appreciate the warm setting at low light levels for creating nice and cozy ambiances [45], but this
can be different per region. Some Philips branded EyeComfort LEDs of Signify provide the WarmGlow
dimming feature. By combining two different LEDs (2200K and 2700K) an incandescent dimming
behavior can be mimicked. The WarmGlow feature comes in two variations. SceneSwitch with fixed
settings and smooth WarmGlow dimming over the whole range. (2700K-2200K).
Next to the ambiance effect, a dimming feature combined with a CCT change also has advantages
regarding the circadian rhythm of people. Our biological clock tells us when to wake up and when to fall
asleep. The intensity and action spectrum of light is one of the parameters controlling those responses
[46]. High intensity light that contains a lot of blue makes us feel awake and alert, while low intensity
light with low quantity of blue triggers the release of the sleep hormone melatonin, which makes us
sleepy. Research has showed that bright lighting with a strong blue component is advised in the morning
to support waking up and should be avoided in the evening, because it suppresses the melatonin
production and makes it harder to fall asleep. Dimmed and warm CCT environments in the evening are
ideal for an undisturbed biological rhythm [46].
Philips branded EyeComfort LEDs of Signify with WarmGlow dimming feature support both the
ambiance function and the circadian rhythm of people.
6. Color rendering
Color quality relates to the preference and appreciation of users’ perception of lighting in a given
application. Color quality of white light sources impacts space, objects and human appearance. Poor
color quality can reduce visual discrimination and the accurate rendering of illuminated spaces, objects,
or people. For instance, human skin tones, plants, and foods may appear dull or undersaturated under
lighting with low color rendering and/or low color saturation.
Color rendering of a white light source is defined as the effect of an illuminant on the color appearance
of objects, by conscious or subconscious comparison with their color appearance under a reference
illuminant [47] The general color rendering index (CRI-Ra) is used to measure and specify the color
rendering ability of a white light source, based on a set of eight specific CIE 1974, moderately saturated,
test-color samples (TCS). A CRI of 100 means that the rendering of colors under the test source is equal
compared to the rendering of colors under the reference source (reference being incandescent for CCTs
<5000K)
The preference of users is not always coupled directly to the CRI value. A higher CRI source is not always
more preferred. Color saturation (vividness), especially red saturation, also plays an important role in
preference [48,49,50]. Some over-saturation is in general preferred by people, because objects look
more colorful. The preference for skin tone appearance is different, also between cultures.
It is important to find the right balance between color fidelity (CRI) and color saturation for a specific
application. Philips branded EyeComfort LED of Signify aims to improve color differentiation and
enhance aesthetics through the use of LEDs with good color quality properties.