FieldSpec^® Foreoptics

The FieldSpec^® fiber optic cable provides the flexibility to adapt the instrument to a wide range of applications. One may choose to perform field measurements while handholding the FieldSpec^® fore optics or mount the pistol grip and other fore optic accessories to a tripod.

Typically, reflected radiance and surface reflectance measurements are made using the hand-held configuration. When required for more precise orientation, the FieldSpec^® pistol grip is available with both a sighting scope and leveling device. These accessories allow the user to view the exact spot where the fore optic is pointed, and allow the fore optic to be oriented in precise, nadir-viewing, geometry. Because of the need to orient the irradiance collector in precise geometry, the majority of irradiance measurements are performed using the fore-optics mounted on a tripod.

The small size of the fore optics greatly reduces error associated with instrument self-shadowing. Even when the area viewed by the fore optic is outside the direct shadow of the instrument, the instrument still blocks some of the illumination (either diffuse skylight or light scattered off surrounding objects) that would normally be striking the surface under observation. Thus, the instrument (as well as other objects — including the user) should be placed as far as possible from the surface under observation. This orientation requirement also applies to collecting white reference measurements.

The figure above shows the available fields-of-view (FOV) for the FieldSpec^® FR with an instrument fore-optic height of 135 cm. The dashed circle represents the FOV of a non-ASD instrument with a fixed 3° FOV. The solid circles are for ASD's FieldSpec^® FR. The largest circle is the FOV of the FieldSpec^®'s standard built-in fiberoptic cable, with optional foreoptics providing 1°, 5°, 8°, or 18°. Fore optics covering approximately the same range of angular FOVs are available for the other FieldSpec^® instruments.

Spectral Radiance

The FieldSpec^® also measures ‘spectral radiance’ for contiguous discrete narrow band intervals of one nanometer and the units of spectral radiance are, Watts/meter2/steradian/nanometer. Either the bare fiber optic cable or a variety of optional Field-of-View Lens Foreoptics can be used with the ASD FieldSpec^® for measuring radiance.

Cosine Receptor

If a radiometer is to measure radiant energy in irradiance units, then the sensor surface must emulate a response similar to the example above, and it must integrate all such ‘beams’ of radiant flux incident on the surface as in the integral shown above. In order that the sensor surface emulates this ‘cosine’ response, various designs of optical accessories have been created, which are located between the incident radiant energy and the sensor surface. Such accessories are typically called ‘cosine receptors’, or ‘cosine collectors’. The form of ASD’s cosine receptor is referred to as a diffusion-disc collector (DDC).

A DDC is constructed of a tube with one end covered by a diffusion-disc. Both the tube and disc are designed with a geometry and material that optimizes the cosine response. The radiometer sensor views the radiant energy that transmits through the diffusion-disc.

The ASD Remote Cosine Receptor (RCR) is designed to interface with the FieldSpec® fiberoptic input such that the instrument has a cosine response for measuring irradiance within the geometric hemisphere above the diffuser.

Spectral Irradiance

The FieldSpec^® measures the integrated irradiance for many contiguous discrete wavelength channels, all within a single scan. This kind of irradiance measurement is called ‘spectral irradiance’ since the discrete values are stored and displayed as a spectral plot of irradiance versus wavelength. Spectral irradiance has units Watts/meter²/nanometer, which indicates that discrete irradiance integration, is measured for each discrete narrow-band interval of one nanometer.

Irradiance Observations

ASD has several types of fore optic for irradiance measurements that include:

• ASD's in-air cosine corrected receptors for measurement of total irradiance
• ASD's accessories for measurement of direct irradiance
• ASD's under-water cosine corrected receptors for measurement of in-water up- and down-welling
• irradiance

The FieldSpec Full Sky Irradiance Remote Cosine Receptor can be mounted on a tripod. The base of the irradiance receptor has built-in bubble levels for horizontal alignment of the receptor. The FieldSpec^® fiber optic cable enters the receptor through the gray strain relief fitting.

The Direct Irradiance Attachment fits over the Full Sky Irradiance Remote Cosine Receptor. It acts to limit the angular field-of-view (FOV) of the irradiance receptor. Three interchangeable FOV modules are included for 2°, 1.5°, and 0.5°. FOV aiming sight allows for proper alignment to the solar disk. This accessory is used for measuring the direct component of solar irradiance. Because it uses the same optics as are used for the RCR, relative radiometric errors between total and direct irradiance measurements are minimized.

Artificial Illumination

Sometimes solar illumination is impractical for collecting field spectra or all that is needed is in-situ sample measurements that are not necessarily needed for ground truthing hyperspectral imagery. In those cases, a portable, artificial light source that interfaces with the FR fiberoptic input is necessary. ASD offers two such light sources: the High Intensity Contact Probe and the High Intensity Source Probe.