Ambient and directional dose equivalent

The distinction between H*(10) and H'(0,07) — what each quantity is defined at, which reference radiation quality follows from it, and why selecting the wrong one produces a plausible but invalid result.

Last reviewed 7 September 2026

---

An evaluation begins with a decision that is not a measurement: which operational quantity the instrument is declared for. That decision determines the reference radiation quality, and the reference quality determines the value every relative response is normalised against.

Definitions

Both quantities are defined in the ICRU sphere — a 30 cm diameter sphere of tissue-equivalent material, used as a reference phantom so that a quantity measured in air can be related to dose in tissue.

Ambient dose equivalent, H*(10)

Evaluated at 10 mm depth in the ICRU sphere, in an expanded and aligned radiation field. It is the strongly penetrating quantity: the operational quantity for effective dose from penetrating radiation, and the one used for whole-body monitoring.

The 10 mm depth corresponds approximately to the depth of the organs that dominate effective dose. Radiation that does not reach that depth contributes little to whole-body dose, and H*(10) reflects that.

Directional dose equivalent, H′(0,07)

Evaluated at 0,07 mm depth, in a specified direction. It is the weakly penetrating quantity: the operational quantity for skin dose.

The 0,07 mm depth corresponds approximately to the depth of the basal layer of the epidermis. Beta radiation and low-energy photons deposit most of their energy above that depth and are the dominant contributors to it.

Consequence for the evaluation

The two quantities do not measure the same thing and an instrument is not automatically suitable for both. The distinction propagates through the evaluation in three places.

Reference radiation quality

The reference quality differs by quantity:

Every relative response is divided by the response at that reference, so a different reference produces a different curve from the same readings. IEC 60846-1 is the standard this site currently implements in full, and its energy and angular evaluations are used here as the worked example; the dependence on the declared quantity is not particular to that standard.

Conversion coefficients

The calibration laboratory assigns the conventional true value of the field using conversion coefficients from air kerma to the operational quantity. Those coefficients differ between H*(10) and H′(0,07), and the difference is large at low photon energy. A field certified as 1 mSv/h in H*(10) is not 1 mSv/h in H′(0,07).

Detector construction

An instrument intended for H′(0,07) requires a thin entrance window so that weakly penetrating radiation reaches the sensitive volume. An instrument intended for H*(10) does not, and its housing normally attenuates that radiation deliberately, to approximate the 10 mm depth. The two requirements are in opposition, which is why an instrument that measures both usually does so with two detectors or with a removable window.

Failure mode of a wrong selection

Selecting the wrong quantity does not produce an obviously incorrect result. It produces a complete, internally consistent set of relative responses — for the wrong quantity. Every value is plausible, the verdicts compute normally, and the report is invalid.

The error is normally detected only by someone comparing the report against the calibration certificate. That is why the quantity is worth confirming from the certificate and the instrument specification before booking beam time, rather than inferring it from the instrument type.

Instruments declaring both quantities

Where an instrument declares both, the two are evaluated as separate campaigns with separate reference qualities. They are not two views of one dataset.

In this tool that means two sessions with separate records. Combining them into one session by switching the reference row partway through produces a set of relative responses normalised against inconsistent references, and the verdicts computed from it have no defined meaning.

Operational quantity by instrument standard

Personal dose equivalent, Hp(10) and Hp(0,07), is defined in the body rather than in the ICRU sphere and applies to dosimeters worn by an individual. It is a third operational quantity, and the instrument standard that applies to a given instrument determines which of the three is declared.

Read from the title of each standard as published. Where a title does not name an operational quantity, none is asserted here.

The instruments page sets out which standard applies to which instrument class.