Measurement principle: interferometric volume measurement
The VolumeSense instrument measures the elevation of the liquid meniscus in every well of a microplate with low-coherence interferometry, then turns that elevation into a volume. Measure without contact. Only near-infrared light reaches your sample.
The diagrams are interactive: step through the first, and drag the sliders in the second and last.
Background
How an Interferometer Works
- Probe beam
- Reference beam
- Source and recombined beam
Interference
Two Light Waves Add or Cancel
Waves add
Light is a wave. Where the probe wave and the reference wave meet at the detector, they add together.
Path length difference decides
Crest meets crest and the waves reinforce: bright. Crest meets trough and they cancel: dark. Which one happens depends on the path length difference between the two paths.
Fringes
Changing the path length difference by half a wavelength, a fraction of a micrometer, flips the detector from bright to dark. The repeating pattern of bright and dark is called fringes. Because the wavelength of light is so short, tiny elevation changes can be measured.
Drag the slider to shift the reference wave against the probe wave. In the interferometer, moving the reference mirror does exactly this.
The coherence gate
Low-Coherence Light Locates the Meniscus
Coherence length
Light is emitted in wave trains. The probe and reference waves form fringes only while the phase relationship between them stays sufficiently constant. The range of path length difference over which fringes appear is called the coherence length.
Narrowband light (laser)
A laser has a very long coherence length. Fringes persist over a long range of path length difference, so the signal cannot say where the meniscus is.
Broadband light (low coherence)
Broadband light has a coherence length on the order of micrometers. Fringes appear only while the two paths match to within that length.
The result
The meniscus shows up as a short burst of fringes. The maximum of the burst is the elevation of the meniscus.
The measurement
The Instrument Turns Meniscus Elevation into Volume
The probe beam
The probe beam is aimed down into each well and reflects from the meniscus.
The sample
The sample remains unaffected by the near-infrared light in the probe beam, which is not absorbed. Photodamage is avoided.
The signal
The reflection appears as a burst of fringes, affected only by the elevation of the meniscus and no other sample properties.
The volume
Meniscus elevation plus the geometry of the labware gives volume. No dyes required.
Illustration for a cylindrical well 6.4 mm across with a 3 mm well bottom elevation. Elevation is measured from the bottom of the plate. Real plates use their measured geometry.
Move the slider: the burst follows the meniscus.
Beyond a single read
Evaporation, Well by Well
Because the instrument reads without contact and does not change the sample, the same plate can be read again and again. Repeated reads show how much liquid each well loses to evaporation over time, so you can tell a dispensing error from liquid that has evaporated since.
Talk to us
A video call with the people who built the instrument. Real hardware, real plates, and your questions answered as they come up.
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