- Overview
- Specification
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- Analyses Hemispherical Images
- Calculates canopy structure parameters and solar radiation indices
- Predicts radiation levels beneath canopy
The HemiView canopy image analysis system provides the hardware and software needed to acquire and analyse hemispherical images of forest canopies.
Designed for field use
Images are obtained looking upwards from beneath the plant canopy, or from the forest floor, using a 180° fisheye lens and a high resolution digital camera. Uniform sky conditions are required, such as exist just before sunrise or sunset, or when the sky is evenly overcast. The digital camera is mounted in a Self-levelling Mount to ensure it is held horizontally, whatever the mounting angle. The Mount also provides markers for the horizon and north-south axis, to assist with image alignment.
Powerful image analysis
The HemiView canopy image analysis system processes the image data file to obtain information about canopy structure e.g. Leaf Area Index (LAI), gap fraction and distribution of openings. By combining this information with radiation data, HemiView can predict the PAR intercepted and the radiation levels beneath the canopy. The software also calculates radiation indices, canopy indices, suntrack overlay and the direct and diffuse site factors; advanced graphing facilities help visualise the data. Data can be exported in several common formats.
The leaf area of a single tree can be calculated from HemiView outputs by using an Excel spreadsheet provided with the software.
Constant improvement
The HemiView canopy image analysis system is frequently updated to take advantage of improvements in digital camera technology. The latest evolution, adding an even higher resolution digital SLR camera to the system, significantly improves HemiView’s performance. Resolution can be the key to accurate analysis of hemispherical photographs, which is why we now supply a 20 megapixel SLR CMOS camera. The camera comes complete with a fibre optic flash transfer assembly (to illuminate the north/south image markers) and memory card.
A matching high quality fisheye lens and redesigned camera mount have also been introduced. The camera mount is considerably lighter than earlier versions, with improved the ergonomics. We have also introduced a new protective case that provides a safe and convenient way to store and transport the camera, lens and mount.
Upgrades
Please contact Delta T Devices for information on upgrading older HemiView models to the latest specification.
Leaf Area Index (LAI) in low canopies
HemiView is ideal for analysing and characterising canopy cover in forests and woodland. However, for low regular canopies such as crops, the Delta-T SunScan System is the correct choice.
Specification
| Image file types | BMP, JPG, TIF, Photo CD. |
| Lens distortion | Can be specified as a polynomial relating zenith angle and radial distance. |
| Direct light model | Simple atmospheric transmission, with user defined settings. |
| Diffuse light model | Uniform or Standard overcast sky. |
| Data output | Excel compatible spreadsheet built-in. |
Outputs
| Sky Geometry | Centroid, solid angle and pixel count for each sky sector. |
| Gap Fraction | Proportion of visible sky by sector. |
| Leaf Area Index | By sky sector or overall value. |
| Solar Radiation | Direct and diffuse, above and below the canopy, in energy or molar units. |
| Site Factors | Direct, indirect (diffuse), global. |
| Time series and Sunflecks | Visibility of the solar disc (penumbral effects) and solar irradiance on chosen days, sampled at user-defined intervals or summarised as a sunfleck sequence. |
| Optional cosine correction | For any orientation of intercepting surface. |
| Overall values | Most outputs can be tabulated by sky sector, or aggregated into a single overall whole sky or annual value. |
Features
| Interactive graphical alignment tool | Precise registration of images with the hemispherical co-ordinate system. Compensates for magnetic declination. |
| Responsive image classification | Threshold intensity discriminates visible and obstructed pixels. The classified image view is updated in real-time. |
| Multiple and split views on any image | Classified and colour / grey scale views, or separate sections of a large image can be viewed side by side. |
| Negative images | Photographic negatives can be viewed and analysed as positives. |
| User-defined number of diffuse sky regions | Enables use of custom ranges of zenith and azimuth angle. |
| Leaf Area of a Single Tree | Excel spreadsheet provided with instructions for calculating single tree LAI using HemiView outputs. |
| Analysis of partial images | Unwanted parts of images can be ignored by the analysis. |
| Configurable for site, lens, solar model | User-defined site, lens and solar model characteristics can be applied to any image by selecting from a list. |
| Pre-defined lens characteristics | Lens equations are provided for current and previous lens types supplied by Delta-T Devices |
| Results output to Excel compatible workbook | Additional calculations can be performed using standard spreadsheet formulae. Alternatively, workbooks can be saved for further analysis using Excel. Results can be appended to existing workbooks. |
| User-defined output configurations | The user can define multiple custom output configurations which can be applied to different images. |
Accessories
Hardware
Main components and software
Mountings and enclosures
Installation accessories
Cables and networks
GPRS remote comms
Spares and consumables
Power options
Other
Product Resources
Data Sheets
- HemiView HMV1-DS-9 data sheet 1 MB
Filename: hmv1-ds-9_ver-2
Manuals
FAQs
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How can I build a lens calibration jig?
Question
How can I build a lens calibration jig?
Answer
If we have no information about a particular lens, you will have to calibrate the lens yourself.
One way of building a calibration jig is to remove the spokes from a bicycle wheel and mount the camera at the centre. The holes in the wheel rim provide calibration points at regular zenith angle spacings, so you can measure their locations on a photograph and construct an equation which relates the position on the photograph to zenith angle.
In the meantime, you could scan, threshold and align the images, and analyse them using the Linear 180 lens. If you save the image properties as .hvs files, it will be straightforward to later re-analyse the images using the correct Camera Lens equation.
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How can I get site factors ISF DSF and GSF from a particular photo for a particular day or for a range of days?
Question
How can I get site factors ISF DSF and GSF from a particular photo for a particular day or for a range of days?
Answer
ISF: indirect site factor is time-invariant
DSF: you can calculate daily DSF values by summing DirAb and DirBe (or DirAbU and DirBeU) columns in the TimeSer sheet and dividing one by the other
GSF: is defined as (direct + diffuse below) / (direct + diffuse above). To calculate a daily value:
- Calculate direct above and below as for daily DSF (above).
- To approximate diffuse above, start with the annual DifAb (or DifAbU) value in the Values sheet, apportion according to the monthly weighting in the DifMonth sheet, and divide by the number of days in the month.
- Calculate diffuse below by multiplying diffuse above by ISF.
For values spanning a series of days, repeat the above operations for each day and sum the radiation values before calculating the site factors.
For periods of a month or more:
DSF: you can calculate monthly DSF values by taking the appropriate values from the DirAb and DirBe sheets, and dividing one by the other.
GSF: is defined as (Direct + Diffuse below) / (Direct + Diffuse above).
- Take Direct above and below from the DirAb and DirBe sheets.
- Calculate Diffuse above as DifAb annual value multiplied by the appropriate month fraction from the DifMonth sheet
- Calculate Diffuse below by multiplying Diffuse above by the ISF value
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How do I adjust the solar model to match my climate data?
Question
How do I adjust the solar model to match my climate data?
Answer
If you have records of Direct & Diffuse radiation:
The Solar Model requires values for the direct transmissivity, and for the diffuse proportion.
Integrate your data to give monthly totals of Direct and Diffuse radiation.Calculate the equivalent values using HemiView – Direct radiation integrals are given in the Total row at the bottom of the DirAb sheet, Diffuse radiation integrals are calculated by multiplying the DifAb value in the values sheet by the appropriate month value in the DifMonth sheet.
By trial and error, adjust the values of transmittivity and diffuse proportion to give best fit between your measured irradiance at the point, and the computed irradiance using HemiView. You may find that a single solar model setting will not give a good fit for the whole year, so it may be appropriate to find several sets of solar model settings that work at different times of the year, and only use the corresponding outputs for those times.
If you have records of Sunshine duration:
Calculate the percentage sunshine as the number of actual sunshine hours divided by the total possible sunshine hours (sunrise to sunset). Multiply this percentage by 0.9 to give an approximate value for the direct transmissivity.
Set the diffuse proportion to a value of 0.1 for direct transmissivity >0.8 or < 0.2; to 0.2 for direct transmissivity between 0.2 – 0.4 or 0.6 – 0.8; to 0.3 for direct transmissivity between 0.4 – 0.6.IMPORTANT NOTE
If you have distinct seasons with very different radiation characteristics, you will need to do these calculations for each season, so you have different solar model settings for different parts of the year. Only use your results for the period when the solar model is valid. -
My site is on a slope so how do I set up HemiView?
Question
My site is on a slope so how do I set up HemiView?
Answer
Site factors: In general, site factors assume you are measuring radiation on a horizontal surface ( ie radiation measured using properly levelled cosine-corrected sensors) so you should make no adjustment for this.
However, there may be circumstances when you want to look at radiation load on the canopy floor, when it is appropriate to take account of the slope. You can do this by changing the Intercepting Surface settings. The values you have to enter are the Azimuth and Zenith angles of the perpendicular to your surface.
The Azimuth angle is the angle from North (via East) of the steepest downwards slope.
The Zenith angle is the same as the maximum angle between your slope surface and horizontal.
Select a single sided surface (double sided is only appropriate to single leaves that receive radiation from both sides).
Your values of radiation above and below will now be the same as if you measured them with a cosine corrected sensor in the plane of your surface.
Product Comparisons
- SunScan and HemiView LAI Models Compared ver 3
Comparison of SunScan and HemiView LAI Models
DownloadFilename: sunscan-and-hemiview-lai-models-compared-ver-3-26-08-15