Colorimeters are devices used to measure the color of objects. They quantify color using various color systems and models. One such system is the L*a*b* color space. The L*a*b* colorimeter is used to measure color as three values – L* for lightness, a* for green/red, and b* for blue/yellow.
Introduction to L*a*b* Color Space
The L*a*b* color space, also known as CIELAB or CIE L*a*b*, was developed by the International Commission on Illumination (CIE) in 1976. It is based on the opponent colors theory of color vision which states that the human visual system interprets color information as light/dark, red/green, and yellow/blue pairs. The goal of this color space was to create a standardized, device-independent model to quantify color that approximates human vision.
The three coordinates in the L*a*b* model represent:
- L* – Lightness ranging from 0 (black) to 100 (white)
- a* – Green/Red spectrum with green negative and red positive values
- b* – Blue/Yellow spectrum with blue negative and yellow positive values
Unlike the RGB color model which depends on device characteristics, L*a*b* is an absolute colorspace based on human perception. It contains all perceivable colors, is device-independent, and is useful for applications like color management, color matching, and image processing.
Principle of Operation
The L*a*b* colorimeter works on the principle of tristimulus colorimetry. It uses three filters to approximate the response of the cones in the human eye which sense red, green and blue color. The filters used are:
- X – approximates x-bar, response to red light
- Y – approximates y-bar, response to green light
- Z – approximates z-bar, response to blue light
The L*a*b* colorimeter measures the tristimulus values X, Y and Z of the test sample. These values are then converted into the L*, a* and b* coordinates using mathematical transformations:
L* = 116(Y/Yn)^(1/3) – 16
a* = 500[(X/Xn)^(1/3) – (Y/Yn)^(1/3)]
b* = 200[(Y/Yn)^(1/3) – (Z/Zn)^(1/3)]
Here, Xn, Yn and Zn are the tristimulus values of the reference white point which is Illuminant D65 in L*a*b*.
Components of L*a*b* Colorimeter
The main components of a typical L*a*b* colorimeter are:
- Light source – Illuminates the sample, often a tungsten lamp or daylight simulating LEDs
- Filters – X, Y, Z filters to simulate CIE standard observer
- Lens – Focuses light reflected from sample onto detector
- Sample holder – Holds sample in place for measurement
- Detector – Measures filtered light, converts to electrical signals
- Processor – Converts signals to tristimulus values and then L*a*b*
- Display – Shows L*, a*, b* readings
| Component | Function |
|---|---|
| Light source | Illuminates the sample |
| Filters | X, Y, Z filters to simulate CIE standard observer |
| Lens | Focuses light reflected from sample onto detector |
| Sample holder | Holds sample in place for measurement |
| Detector | Measures filtered light, converts to electrical signals |
| Processor | Converts signals to tristimulus values and then L*a*b* |
| Display | Shows L*, a*, b* readings |
Working of L*a*b* Colorimeter
The working principle of the L*a*b* colorimeter is as follows:
- The light source illuminates the sample placed at the sample holder.
- The reflected light is focused by the lens onto the detector.
- The X, Y and Z filters allow only red, green and blue light to pass through respectively.
- The detector measures the filtered light and converts it into three electrical signals corresponding to the tristimulus values X, Y and Z.
- The processor converts the X,Y,Z values to L*, a* and b* coordinates using mathematical transformations.
- The L*, a* and b* values are displayed on the screen.
The sample’s color is completely characterized by the three L*a*b* coordinates. L* indicates lightness, a* indicates green/red component and b* indicates blue/yellow component.
Applications of L*a*b* Colorimeter
Some major applications of L*a*b* colorimeter are:
- Quality control – Checking color consistency in products like textiles, plastics, paints
- Color matching – Matching colors between samples or batches for cosmetics, paints, etc.
- Food science – Measuring color changes during cooking, ripening, storage in food products
- Agriculture – Determining ripeness, flavor by color measurement of fruits, vegetables, meat, etc.
- Forensics – Identifying color shades for forensic analysis
- Medical research – Studying biological samples like blood which have color indicators
- Cosmetic research – Evaluating color, opacity, translucency of cosmetic products
- Printing – Color inspection and control in printing processes
The device-independent nature and human-like perception measurement makes L*a*b* colorimeters useful in many fields.
Advantages of L*a*b* Colorimeter
Some key advantages of L*a*b* colorimeters are:
- Device independent, provides absolute measurement of color
- Models human color perception
- Covers full gamut of visible colors
- Quantifies color numerically with good repeatability
- Allows easy detection of small color differences
- Suitable for measuring reflective and translucent materials
- Useful for samples with visual texture and patterns
- Standardized model, L*a*b* data is compatible across systems
Limitations of L*a*b* Colorimeter
Some limitations of L*a*b* colorimeters include:
- Metamerism – Different spectrum can give same L*a*b* values
- Light source dependence – L*a*b* values vary with different illuminants
- Not suitable for emitting light sources like LEDs, lamps, displays
- Observer angle not considered, suitable only for perpendicular measurement
- Difficult to measure very dark colors near black
- Highly saturated colors not measured accurately
- Expensive compared to RGB colorimeters
Conclusion
The L*a*b* colorimeter is a versatile device that provides standardized, absolute color measurement similar to human perception. It is useful for color inspection and control across many industries. With device-independent output and coverage of all perceivable colors, it offers distinct advantages over RGB colorimeters. However, factors like metamerism, light source dependency and angle viewing restrict its usage in certain applications. Overall, L*a*b* colorimetry is a valuable technique for accurate and consistent color measurement.
