Detectors: Silicon Photodiode vs. CMOS Array vs. InGaAs

After light has been dispersed by a monochromator or transmitted through a sample, a detector converts the photon flux into an electrical signal. The choice of detector technology profoundly affects spectral range, sensitivity, measurement speed, and instrument cost. Three detector types appear across the K LAB product range: silicon photodiodes, CMOS linear image sensors, and InGaAs photodiodes.

Silicon Photodiode

A silicon photodiode is a p-n junction semiconductor device. Photons absorbed in the depletion region generate electron-hole pairs that produce a photocurrent proportional to incident light intensity. Silicon responds across roughly 190-1100 nm, covering the entire UV-Vis range with a single element. It is low in cost, compact, thermally stable, and offers a wide linear dynamic range — typically six or more decades. The K LAB Alpha double-beam, POP single-beam, and ProTecUV inline photometer all use silicon photodiodes as their primary detectors. In the Alpha, two matched photodiodes measure reference and sample beams simultaneously, cancelling lamp noise in real time.

CMOS Linear Image Sensor (Array Detector)

Rather than a single-element detector paired with a scanning monochromator, an array detector captures an entire wavelength range simultaneously. A CMOS linear image sensor consists of a row of hundreds to thousands of individual photodiodes on a single silicon chip, each registering a different wavelength after light has been dispersed across the array. The K LAB NanoQ microvolume spectrophotometer uses a 2048-element CMOS linear image sensor, which records the complete spectrum from 190 to 850 nm in a single xenon flash — no moving parts, no scan time. Array detection is also inherently more resistant to lamp flicker because all wavelengths are captured in the same instant.

Scanning (single detector) Grating scans Slit PD One wavelength at a time Sequential scan required Array (CMOS / InGaAs) Fixed grating All wavelengths at once Single measurement instant Si Photodiode 190-1100 nm, low cost, scanning or fixed CMOS Array 190-850 nm, 2048 pixels, snapshot InGaAs 900-2500 nm, NIR specialist, TE-cooled
Single-element photodiodes scan one wavelength at a time; array detectors capture the full spectrum in a single exposure.

InGaAs Photodiode

Indium gallium arsenide (InGaAs) is a III-V semiconductor with a bandgap tuned to absorb near-infrared photons that silicon cannot detect. Standard InGaAs devices respond from about 900 nm to 1700 nm; extended InGaAs covers up to 2500 nm. This makes InGaAs the detector of choice for NIR spectroscopy applications such as raw material identification, moisture analysis, and protein content measurement. The K LAB MRX N1 FT-NIR analyser uses a 2-stage thermoelectrically cooled InGaAs PIN photodiode. Cooling reduces thermal dark current, which would otherwise degrade sensitivity at longer wavelengths where photon energies are low.

Choosing the Right Detector

The application dictates the detector. Silicon photodiodes are the universal choice for UV-Vis work — reliable, affordable, and spectrally well-matched. CMOS arrays add speed and robustness for microvolume and high-throughput scenarios. InGaAs is reserved for NIR measurement where silicon is simply transparent to the wavelengths of interest. Understanding these trade-offs is the first step in selecting an instrument matched to the analytical task.

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