Sequoia Scientific


High-Precision Transmissometer

Optical Transmission and Beam Attenuation

LISST-Tau is a high-precision transmissometer for underwater vehicles, profiling packages, CTDs, and other systems. With high-quality optics and a carefully selected LED source, it transmits a collimated beam through the water, and precisely measures the light transmitted through its 15 cm path. Light modulation with synchronous detection rejects ambient light, while oversampling and averaging yield better than 16-bit resolution. LISST-Tau’s digital controller applies calibration coefficients, corrects for temperature effects, and transmits data from both digital and analog outputs. Included software provides real-time display of data, and functions for checking and updating pure-water calibrations. An optional flow-through chamber simplifies calibrations and experiments. LISST-Tau advances the state of the art for in-situ transmissometers.

NOTE: The LISST-Tau is restricted in its use and can ONLY be sold for civilian and scientific purposes. ALL PO’s for LISST-Tau must be accompanied by a completed end-use statement.


  • Measures optical transmission and beam attenuation
  • Externally powered
  • RS-232 and analog real-time outputs
  • Ambient light rejection
  • Onboard temperature compensation
  • Available with green or red LED source (specify upon ordering)


Parameters Measured

  • Optical transmission
  • Beam attenuation

Operating Ranges and Stability

  • Operational temperature range: -3 °C to 40 °C
  • Storage temperature range: -20 °C to 60 °C
  • Beam attenuation range: ~0 m-1 to 30 m-1
  • Linearity (concentration): >99 %
  • Short-term stability (1 min standard deviation)
    • Transmission: ~0.002 %FS
    • Beam attenuation: ~0.00013 m-1
  • Long-term stability (6 hr test)
    • Transmission: ~0.003 %FS/hr
    • Beam attenuation: ~0.00016 m-1∙hr-1


  • Optical path length: 15 cm
  • Source wavelength: ~532 nm (green) or ~650 nm (red) LED
  • Source spectral bandwidth: <10 nm FWHM
  • Acceptance angle (half angle, in water): 1.0 °
  • Optical transmission @ 16-bit resolution

Mechanical and Electrical

  • Dimensions [Ø x L]: 5.1 cm x 40.6 cm (2.00″ x 16″)
  • Weight [air / seawater]: 1.140 kg / 0.585 kg (2.5 lbs / 1.3 lbs)
  • Depth rating: 2,000 m
  • Sampling rate: Up to 1 Hz (default)
  • External power input: 7 VDC to 25 VDC
  • Current drain @ 12V: 42 mA average during sampling
  • Connector: SubConn MCBH6M


LISST-Tau Item # and Description

Item #DescriptionThumbnail
SEQ-FP-LTAU-532NMLISST-Tau 532 nm (green) 0.15 m path length transmissometer


SEQ-FP-LTAU-650NMLISST-Tau 650 nm (red) 0.15 m path length transmissometer


LISST-Tau Accessories *INCLUDED* with LISST-Tau

Item #DescriptionThumbnail

LISST-Tau Accessories *NOT INCLUDED* with LISST-Tau

Item #DescriptionThumbnail
SEQ-AC-LTAU-ZINC10Replacement zinc anodes for LISST-Tau (bag of 10 anodes)Picture on the way!
SEQ-AC-LTAU-ZINCReplacement zinc anode (bag of 01 anode)Picture on the way!
SEQ-AC-LTAU-PT6FMCIL6F//MCDLS-F//0.6 m - open ended
6-Position Pigtail 24" only w/Lock Sleeve
Picture on the way!
SEQ-AC-LTAU-EW1-year extension of LISST-Tau standard warrantyNo picture
SEQ-AC-LTAU-SERV3-year LISST-Tau service planNo picture


Can the LISST-Tau be ordered with different optical path lengths?

No. The LISST-Tau is offered only with a 15 cm (0.15 m) path length.

Theoretically, a long path length transmissometer (say, 1 m) would enable measurements of transmission values >> 0.999. However, Sequoia has found that the practical implementation of a long path length transmissometer is hampered by mechanical issues, e.g., difficulties in keeping the instrument aligned under pressure. By and large, these mechanical issues offset the theoretical advantage one would expect from the longer path length.

If you want to measure beam attenuation in waters where a path length < 15 cm is required, Sequoia recommends the LISST-200X or LISST-Glider.

Why is the instrument called LISST-Tau?

LISST was originally an acronym for our laser particle sizing sensors. LISST now is just a Sequoia trade mark.

Tau is the name of the Greek letter τ. In marine optics, τ has traditionally been used to denote optical transmission, the fundamental measurement made by the LISST-Tau.

Beam attenuation, c, is related to optical transmission, τ, thus:

c = (1/r) · ln(τ), where r is the optical path length in m (0.15 m for LISST-Tau).