Spectral Workbench 2 Tutorial: Capturing and Analyzing Light Spectra

An absorption spectrum is a continuous spectrum with dark lines formed when white light passes through a gas or transparent material, as the atoms or molecules absorb specific wavelengths of light; conversely, an emission spectrum is a collection of bright lines at specific wavelengths emitted by atoms when they transition from higher to lower energy states, and these two spectra are complementary phenomena where the dark lines in absorption correspond to the bright lines in emission.

When light passes through a prism, white light creates a continuous spectrum containing all colors, but when light from excited hydrogen gas passes through a prism, it produces a line emission spectrum with only specific colored lines characteristic of hydrogen; conversely, when white light passes through hydrogen gas before entering the prism, it creates an absorption spectrum with dark bands at the same positions as the emission lines, and each element has a unique spectral fingerprint that serves as its chemical identifier.

A spectrum is the separation of white light into seven colors (VIBGYOR) with different wavelengths. In absorption spectrum, when white light passes through a substance like sodium chloride solution, certain wavelengths are absorbed by the atoms, causing dark lines to appear on the screen where those colors are missing. In emission spectrum, when hot gases absorb energy and then release it, they emit specific wavelengths that appear as colored lines on the screen. The key difference is that absorption spectrum shows dark lines where light is absorbed (atoms move to higher energy levels), while emission spectrum shows bright colored lines where light is emitted (atoms return to lower energy levels).

Emission and absorption spectra occur when electrons in atoms absorb or emit photons with discrete amounts of energy corresponding to the differences between energy levels, following the principle of conservation of energy; scientists use these spectral fingerprints to identify the elements present in a material by analyzing the specific wavelengths of light that are absorbed or emitted.

This section covers the classification of spectra into emission and absorption types. Emission spectra are produced by glowing or excited materials and are classified into three types: continuous spectrum (continuous range of wavelengths without gaps), line emission spectrum (discrete bright lines on dark background), and band emission spectrum (groups of closely spaced lines appearing as bands). Absorption spectra are produced when light passes through a cooler gas or material, creating dark lines or bands in the continuous spectrum. The dark lines correspond to wavelengths that the material would emit if it were glowing. This classification is fundamental to understanding how different materials interact with light.

Digital cameras capture light using sensil (sensing elements), not traditional pixels. The Bayer filter system uses four color filters—two green, one red, and one blue—arranged in a pattern. Each filter allows only matching light to pass through, and the camera processor calculates color based on which sensil received light. Every visible pixel actually involves nine sensil: one central and eight surrounding. Basic colors come directly from single-channel activation (red, blue, green), while composite colors form from combinations (yellow from red+green, aqua from green+blue, purple from red+blue). Absence of light creates black, full saturation creates white, and equal channel values produce grays.

RGB (Red, Green, Blue) are the three primary color channels used by digital camera sensors to capture and reproduce colors, forming the foundation of digital color imaging technology.

RGB channel histograms show the distribution of each color channel separately, revealing color-specific exposure issues that luminance histograms cannot detect. A scene with no green or blue but only red pixels shows a luminance histogram with a gap at 30% brightness, yet the red channel histogram shows a complete gradient. Individual color channels can be overexposed even when the overall luminance histogram appears normal. Red flowers can cause the red channel to clip while the luminance histogram shows proper exposure, resulting in loss of detail in red areas. The RGB histogram reveals channel-specific overexposure that luminance histograms cannot detect. Different histogram types serve different purposes: luminance histograms show apparent brightness, RGB histograms combine channels into a single representation, and color histograms show cyan, magenta, and yellow regions. Color channels can be overexposed even when luminance histograms appear normal. Red flowers can cause the red channel to clip while the luminance histogram shows proper exposure, resulting in loss of detail in red areas. Solutions include exposure bracketing (taking multiple exposures), reducing contrast or saturation in post-processing, using graduated neutral density filters, or spot color techniques. No perfect solution exists, as recovering data typically reduces image quality. Cameras have fixed dynamic range determined by sensor technology, with pixels beyond the maximum capture range appearing completely white (clipped). Modern SLRs achieve 10-12 stops of dynamic range, while compact cameras achieve only 5-8 stops. The human eye has an instantaneous dynamic range of approximately 10,000:1 (13 stops) in bright daylight, far exceeding camera capabilities. Camera dynamic range can be understood through a bucket analogy: each pixel is a bucket collecting light. The minimum detectable signal is the smallest amount of light the sensor can measure, while the maximum is the point where buckets overflow (clipping). Larger sensors (SLRs) have better dynamic range than smaller sensors (compact cameras) because larger pixels can collect more light.

This comprehensive section covers the foundational concepts of digital photography color science. Color Space (색 공간) defines how colors are created using specific materials: RGB for light-based displays (white at 100% light, black at no light) and CMYK for printing (black at all inks, white at no ink). Color Gamut (색역) establishes the boundary of color saturation expressible within a color space, with Adobe RGB (1998) offering wider gamut than sRGB. RGB values like R255 are relative, not absolute, meaning the same value can appear differently on different displays. Bit Depth (비트 심도) determines brightness steps: 8-bit provides 256 steps, 10-bit provides 1024 steps, enabling smoother gradients and more detailed transitions. Higher bit depth is preferred when storage allows, as it provides more data for image processing.

All digital photos are composed of three primary color channels: red, green, and blue. These channels function like colored panes of glass with varying transparency levels. When no color is present (all channels at 0%), the result is black. When all three channels are fully transparent (100% opacity), the result is white. Intermediate opacity levels create all the colors visible in the color spectrum. Understanding these three fundamental channels is essential for mastering color manipulation in photo editing software.

Fluorescent lamps contain mercury arc lamps with fluorescent coatings on the inside of the tube that convert ultraviolet light to visible light. Three strong mercury lines exist in the visible spectrum (at 435.8 nm, 546.1 nm, and 577-579 nm) that are so repeatable they are used to calibrate spectrometers. Manufacturers control color temperature by changing the mixture of materials in the fluorescent coating. While the Little Garden has better spectral resolution, the TLM-2 shows better spectral accuracy with all peaks identified within 1 nanometer of reference values from standard tables.

Calibration of a DIY spectrometer can be done without expensive equipment using a fluorescent lamp. These lamps contain mercury, which emits two characteristic and stable spectrum lines at 546 nm and 436 nm. By using these known reference points, the device can show other wavelengths with good precision.

Fluorescent lamps contain mercury vapor that emits light at specific wavelengths when excited. The captured spectrum shows characteristic peaks in the violet, blue, and green regions, with significant gaps in between. These emission lines are the signature of mercury and are used to calibrate the spectrometer.

Calibration is defined as a set of operations that establishes, under specified conditions, the relationship between values indicated by a measuring instrument or system and corresponding known values realized by standards. The purpose of calibration is to ensure that instruments provide accurate measurements by comparing them against reference standards traceable to international measurement systems. For example, when purchasing a new micropipette or balance, calibration ensures that the device actually measures what it claims to measure—1 ml truly equals 1 ml, or a thermometer accurately reads temperature values. This comparison with international reference standards creates a chain of traceability that validates measurement accuracy.