Vitis 57, 159–165 (2018)

DOI: 10.5073/vitis.2018.57.159-165

Evaluation of photosensitive films for light measurements in the fruiting zoneof grapevine canopies
A. Bontempo1), 2), J. Smith1), M. Brandt1), E. Peterlunger2) and M. Stoll1)¹) Hochschule Geisenheim University, Department of General & Organic Viticulture, Geisenheim, Germany²) Department of Agricultural, Food, Environmental and Animal Sciences, University of Udine, Udine, Italy

Summary
The potential influence of fruit exposure and canopy manipulations on grape berry composition is wellrecognized. However, a simple and low cost method forquantifying the amount of light reaching the fruitingzone is lacking. The objective of the present study was
to test the application of a commercial system of photosensitive azo-dye coated plastic films for characterizing
light conditions within grapevine canopies. The fading
rates of three films of differing light sensitivity were
initially monitored in a fully exposed position, and these
all showed a linear or near linear relationship with radiation measured by an adjacent global solar radiation
sensor. When mounted in the fruiting zone of a verticallytrained cool climate 'Riesling' vineyard for two periods
following early and late leaf removal treatments, the films
were able to quantify the change in light exposure within
the fruiting zone. Total radiation values calculated using
an on-site calibration or manufacturer´s equation werecomparable. While some consideration is needed withregard to the choice of film sensitivity and positioning
within the canopy, these initial evaluations suggest theselight sensitive films can provide a simple and accurate
method for characterizing light conditions and quantifying cumulative radiation within the fruiting zone.
K e y w o r d s : light interception; microclimate; solar
radiation; training system; light sensitive dyes.

Introduction
The management of light interception is an important
aspect of wine grape production, and many viticulture
practices are linked to the control of light interception byfoliage and the internal light environment of grapevine
canopies (Smart et al. 1980). The effect of sunlight exposure on berry composition, and on subsequent wine quality,
is of direct practical interest to grape growers as there issignificant scope to modify the amount and timing of light
exposure through trellis design and within season canopy
manipulations. However, without quantifying the effectsof canopy management practices on fruit light exposure,
it is difficult to make more than empirical links betweenchanges in fruit microclimate and specific responses of fruit
and wine composition.
Individual sensors or arrays of sensors can be used for
direct light measurements, but to integrate light exposure
over longer time periods, and provide sufficient measurement points to overcome heterogeneity, multiple sensors andassociated data logging equipment are required (eg. Casadesus et al. 2011). An alternative approach to quantifying
solar radiation involves the use of light sensitive dyes. Priceet al. (1995), for example, demonstrated the application oflight sensitive papers (Sunprint Kit, University of California,
Berkeley) in grapevine canopies with rolled tubes placedin the fruiting zone of vertically trained and minimallypruned canopy systems for periods of 5 min. The pattern ofcolour change after development was used to characterizepotential differences in bunch light exposure within thetwo canopy types, but the high light sensitivity of the paper
would suggest it is less suited for the characterization of
canopy light conditions over more extended periods. For
longer term measurements, an alternative approach has been
to stack sheets of diazo paper and calibrate total radiation
received against the number of layers exposed (Friend 1961).
Although it has subsequently been shown that the exposed
layer number correlates more closely with light intensity thancumulative radiation exposure (Bardon et al. 1995), otherauthors still report useful correlations with instrument based
canopy and light measurements, and suggest the method
is still of value for characterizing canopy effects on light
exposure where other options are not available (Buckley
et al. 1999).
A second class of compounds that have been used toquantify light exposure are the azo dyes. These pigments
undergo photo-decomposition to colourless bi-products,
and when coated on transparent plastic films the change inabsorbance following light exposure can be measured using
a spectrophotometer (Yoshimura et al. 1990). The dyes arealso water-fast, and following an initial demonstration ingreen algae containing water by Yoshimura et al. (1990),
an early application of the films was the characterization ofunder water light environment around mangroves (Hirano
et al. 1996). Subsequent studies have seen these films used

Correspondence to: Dr. J. Smith, Hochschule Geisenheim University, Department of General & Organic Viticulture, Von-Lade-Str. 1,
65366 Geisenheim, Germany. Fax: +49-(0)6722-502-140. E-mail: jason.smith@hs-gm.de© The author(s).

This is an Open Access article distributed under the terms of the Creative Commons Attribution Share-Alike License(http://creative-commons.org/licenses/by-sa/4.0/).

A. Bontempo et al.

160

to measure variability in greenhouse light distribution(Fitz-Rodríguez et al. 2011), light distribution under forest
canopies (Kawamura et al. 2005), modelling light interception of tomato leaves (Higashide 2009), and measuring thelight exposure of apples as a predictor of fruit temperature(Yamada et al. 2004).
The objective of the present study was to test a viticulture application for the current commercial system of thesephotosensitive azo dye based films. This system consistsof a hand-held spectrophotometer and three different film
options ranging in sensitivity from one to seven days to fadein full sunlight. For characterizing light conditions withingrapevine canopies, such films are of potential interest as
they provide a simple, weather resistant and low cost option
for large numbers of cumulative light measurements. In
this study, the fading rate of the three films were comparedagainst a reference pyranometer, and for situations whereusers may not have access to local solar radiation data or
their own sensors, a simple comparison was also made withcalibrations provided by the manufacturer. In addition, themedium sensitivity film was then used in vertically trained'Riesling' vineyard to determine if changes in bunch light
exposure following standard cool climate leaf removal
practices could be quantified with films mounted in the
fruiting zone.
Material and Methods
L i g h t s e n s i t i v e f i l m s : The "OptoLeaf"
light sensitive films used in the study were supplied by
Taisei-Environmental & Landscape Group, Tokyo, Japan.
Films of three differing light sensitivities are available, andare identified according to the colour of their respective
dyes and approximate fading time in full sunlight. Theseare "O-1D", the most sensitive orange 1 day film with
an absorbance maximum at 493 nm, "R-3D" the medium
sensitivity red 3 day film with an absorbance maximum at
521 nm, and "Y-1W" the least sensitive yellow 7 day filmwith approximately one week of exposure time available
in full sunlight and absorbance maximum at 468 nm. The
absorbance spectra of each film, together with that of theblank tricellulose acetate backing films, is shown in Fig. 1.
F i l m c a l i b r a t i o n s : Two outdoor calibrations
were undertaken with samples of the three film types andcompared with calibration equations provided by the manufacturer in their supporting printed material (unpublished).
The manufacturer calibrations provide an option for conversion to global solar radiation (MJ·m-2) or photosynthetically
active radiation (µmol·m-2·s-1), and for each film there isan equation for winter, summer and spring/autumn. As therate of fading is influenced by ambient temperature, and theprovided calibrations were produced locally in Japan, themanufacturer suggests using an on-site calibration if higher
accuracy is required. For this reason we did not make any
direct statistical comparison with their equations and ours.
However, as site specific calibrations may not always be
possible, we do present a comparison of radiation values
calculated from the manufacturers´ summer calibration with
our own version in the subsequent field component of thisFig. 1: Absorbance spectra of the three light sensitive film types,
and blank tricellulose acetate substrate.

study. For each calibration, each film type was cut into four
replicates pieces of 20 x 35 mm and mounted in re-useable
plastic slide frames (Kaiser Fototechnik, Buchen, Germany).
An aluminium stand with horizontal level adjustment was
built to support the mounted films at a height of 110 cm,
and then placed 3.3 m from a weather station located at theGeisenheim University campus. Readings were made at regular intervals with the portable spectrophotometer (D-Meter
RYO-470 Taisei-Environmental & Landscape Group, Tokyo,
Japan) until the films faded below 0.6 absorbance units. The
absorbance of films below the readers range were recordedwith a laboratory spectrophotomer (SPECORD® 50 PLUS,
Analytik Jena, Germany). In accordance with the manufacturer’s instructions, the film absorbance was then expressed
as a percentage of initial absorbance for use in subsequentcalibrations. Radiation received between each measurement
interval was calculated from the global solar radiation data
recorded by the weather station pyranometer at 15 min.
intervals (CMP6, Kipp and Zonen, Delft, Netherlands).
UV filter, orientation and backgrounds c r e e n i n g : To test the influence of film orientation,
UV screening, and background reflected radiation, three
additional calibration comparisons were conducted with the
R-3D film. The first calibration compared films mounted
horizontally and according to the manufacturer's instructions(dye side facing the sun) with films mounted in reverse
so the triacetylcellulose backing film faced the sun. An
additional set of films was screened with a UV filtering
plastic with a cut-off at 400 nm. The second calibrationcompared films mounted in the recommended orientation
to the sun with films that had either a black or white shade
cloth background placed immediately under the films. The
white shade cloth grade was specified to block up to 60 %of photosynthetically active radiation and the black cloth
80 % (TVV Transport- und Versand-Verpackungen UG &Co. KG, Nattheim, Germany).
F i e l d e v a l u a t i o n s : The intermediate sensitivity
R-3D film was selected to compare fruiting zone light exposure following leaf removal treatments in an established'Riesling' vineyard during 2016. Early and late leaf removal
treatments were applied on July 5 and August 25 respective-

Evaluation of photosensitive films for light measurements in the fruiting zone
ly, with the manual removal of approximately 75 % of leaves
from the fruiting zone on both sides of the canopy. Basedon measurements from the ground to mid-bunch height on arandom selection of vines, the fruit was distributed between
81 and 126 cm, with 50 % of the bunches between 96 and110 cm. The trial was arranged in a randomized block
design with four replicates, and 18 vines per treatment. A
third group of non-defoliated vines served as the control forboth defoliation treatments. The field site was located near
Rüdesheim am Rhein (49°59'20" N, 7°55'56" E), approximately 1.7 km west from the weather station used for thefilm calibrations. The vines were planted at a spacing of 105
cm with 205 cm between rows and pruned to a single cane(10-12 nodes) and trained with a vertical shoot positioned(VSP) trellis system to a canopy height of 220 cm aligned
in a north-south orientation.
The films were deployed between July 29 and August 11
to compare the control and early defoliation treatments, andagain between August 25 and September 12 to compare thecontrol with both defoliation treatments. The absorbance ofeach film was recorded prior to exposure, and then measurements were made in the field to record the progressive rateof fading with the portable spectrophotomer. The films werepositioned within the canopy using 6 mm diameter metal
grapevine training rods that were pushed into the groundbelow the vine and clipped to a fixed trellis wire. Metal clips

161

were attached to the top southern side of each rod, and thenused to hold the film pieces horizontally in the fruiting zone
at a height of 108 cm. The location of the rods was adjusted
slightly if needed to avoid direct shading from bunches but
any leaves in the immediate vicinity of the films were left inposition. Based on measurements from the ground to midbunch height on a random selection of vines, the films wereon average located just below the third quartile of the fruitingzone at 110 cm. The films were installed at every second
vine, giving 9 per replicate and 36 in total per treatment.
On each occasion, sets of films were also exposed adjacentto the weather station to produce concurrent calibrations as
described in the previous section.
For the first deployment period, additional measurements were made with a plant canopy analyser (LAI 2200,
LiCOR Biosciences, Lincoln, Nebraska, USA) to characterise the potential light exposure at each film position. In
diffuse light conditions on three evenings between August
1 and August 8, a reading was made with the optical sensor
held immediately above the position of each film in the trial.
A 180° view cap was installed such that only the canopy visible perpendicular and to the south of the film was in the fieldof view of the optical sensor. The transmittance values foreach of the 5 rings were used to provide an independent assessment of the directional effect of the defoliation treatment
on film exposure, and the diffuse non-interceptance (DIFN)
value was used to compare to the film radiation values. The
DIFN value ranges from 0 to 1 (range from no sky visible
to no foliage visible) and only requires the assumption ofno light scattering by the canopy. Statistical analysis wasundertaken with Genstat V18 (VSN International, Hemel
Hempstead, UK).
Results

Fig. 2: Change in absorbance in relation to measured global solar
radiation during the two calibration periods for each film withmean daytime temperature during each calibration indicated in thelegend (a), and comparison with manufacturer's calibrations afterconversion to fading percentage and removal of values below 0.6
absorbance units (b).

C a l i b r a t i o n c o m p a r i s o n s : Above therecommended absorbance threshold of 0.6 absorbanceunits, the rate of colour loss from all three films exhibiteda near linear relationship with the amount of global solar
radiation measured at the weather station (Fig. 2a). For the
O-1D and Y-1W films the relationship was still linear at the
final readings of 0.4 and 0.5 absorbance units respectively,
suggesting that values below the recommended absorbancecould be used and included in calibrations. Under the partly
cloudy conditions that were experienced during the two
calibration periods, the O-1D faded in just over 1 day, theR-3D between 4 and 5 and the Y-1W films between 8 and
9 days. When converted to a relative fading value, withthe remaining absorbance expressed as a percentage of thestarting value (F%), and removing data for films below 0.6
absorbance units, the film responses to intercepted radiationin the present study were comparable with the manufacturers
summer calibration for each film type (Fig. 2b). Although astatistical comparison has not been made with the manufactures calibration, the rate of fading for both the O-1D and
R-3D was slightly, but significantly faster for the two warmer
calibration runs. The fading response of the Y-1W films,
which was best explained by a second degree polynomial,
did not differ between the two calibration runs.

A. Bontempo et al.

162

F i e l d e v a l u a t i o n s : When the films were positioned within the fruiting zone in the field trial following
the early defoliation treatment, the absorbance of the most
exposed film fell to 0.53 and below the recommended valueof 0.6 absorbance units by August 8 (Fig. 3c). Further films
faded below this value when the final measurement wasmade on August 11 (not shown), while a slight unexplained
increase in absorbance was observed for some films. Cumulative radiation values were therefore only calculated until
August 8, although the single value below 0.6 was in thiscase retained in the data-set. Using a calibration equation
derived from films that were exposed between July 27 andJuly 31, which included a set of readings with a mean absorbance of 0.54, the amount of radiation measured in the
fruiting zone of the non-defoliated treatment was 16 MJ m-2
compared to 21.1 MJ m-2 for the early defoliation treatment(Tab. 2). For our calibration the relationship between film
fading and cumulative radiation could be adequately described by simple linear regression forced to a y-intercept(F%) of 100, but the calculated value was otherwise very
similar to the radiation values obtained using the manufacturers summer calibration. Although there was no significant
difference between the two treatments, these values represented 9.3% and 12.2% respectively of the 172 MJ m-2 ofradiation recorded at the weather station.
The initial rate of fading following the second deployment was more rapid with less overcast skies, and from thethird measurement date onwards a number of films in more
exposed positions needed replacing. By September 12, thishad extended to exchanging 52 out of the total 108 films andit became difficult to manage the field spectrophotometerreadings such that films could be replaced before excessively
fading. As for the first deployment, the calculations havetherefore been restricted to shorter period from August 25

Ta b l e 1
Effect of film orientation, UV filtering and background screen
colour on film fading
Film orientation and UV filtering
Controly = -0.0232x+1.85a-

Reversedy = -0.0234x+1.86

nsb

UV filtery = -0.0128x+1.85

P < 0.001

Background screen colour
Controly = -0.0218x+1.77

-

White

y = -0.0246x+1.77

P < 0.001

Blacky = -0.0210x+1.78

P < 0.01

y = absorbance, x = MJ m-2 b difference of slope from control films,
ns indicates not significant.
aMounting films in reverse orientation, with the non-dye
coated surface of the backing film facing the sun, caused a
reduction in the rate of fading (Tab. 1). As the transmission of
UV light through the backing films decreases through 50 %at 310 nm, and is absorbed completely by 280 nm, the small
reduction in the rate of colour loss in reverse orientationsuggests minimal influence of solar radiation below thesewavelengths. However, filtering out all of the UV component below 400 nm with a filter placed in front of the films
slowed the rate of fading by approximately 45 %. When theeffects of reflected radiation were evaluated with different
coloured shade cloth, fading was accelerated above a whitebackground by about 6 % compared to control films. A black
background caused a slight reduction in the rate of fadingof 0.5 % compared to films with no background (Tab. 1).

Ta b l e 2
Effect of early defoliation on July 5, and late defoliation on August 25 on the subsequent exposureof R-3D films mounted in the fruiting zone of vertically trained Riesling. Values show comparison between the manufacturers summer calibration and on-site calibration produced during each
deployment period.
July 29 to August 8
Cumulative
radiation(MJ m-2)

August 25 to September 2

Proportion ofincident radiation(%)

Cumulative
radiation(MJ m-2)

Proportion ofincident radiation(%)

Manufacturers calibration1
Control

15.9

9.2

19.4

12.4

Early defoliation

20.4

11.8

22.5

14.4

Late defoliation

-

-

30.0

19.2

ns

ns

On-site calibration2,3
Control

16.0

9.3

21.0

13.4

Early defoliation

21.1

12.2

24.3

15.5

Late defoliation

-

-

31.5

20.1

ns
1
3

p < 0.05

MJ m-2 = 265.6 - 131.6 × (Log10(F%)), 2 MJ m-2 = (F% - 100)/ - 1.22,
MJ m-2 = (F% - 100)/ - 1.13.

Evaluation of photosensitive films for light measurements in the fruiting zone

163

Fig. 3: Overview of weather conditions and change of film absorbance from deployment following the early defoliation treatment on
July 5 (a,c), and late defoliation treatment on August 25 (b,d).

to September 2 (Fig. 3d). This data set again included somefilms that had faded below an absorbance of 0.6, and four
replicate points with a mean of 0.52 absorbance units were
therefore included in the calibration regression. Despite
higher temperatures during the second deployment, with anaverage maximum of 30.8 °C compared to 24.6 °C in thefirst measurement period, the calculated values were again
very similar for the concurrent calibration (August 26 to 29)
and the manufacturers summer calibration (Tab. 2). For thevalues calculated from our calibration we also found a significant difference between the late defoliation and control
treatment at 21.0 and 31.5 MJ·m-2 for the deployment period.
The total radiation above the canopy was 156 MJ·m-2 with
13.4, 15.5 and 20.1 % received by the films in the control,
early defoliation and late defoliation treatments respectively.
For the canopy light measurements made with the
LAI-2200 during the first deployment, light transmittancemeasured by ring 5 was significantly higher than the control
in the early defoliation treatment, but not at the higher elevation angles from ring 4 to ring 1 (Fig. 4a). There was also
no significant difference in the mean DIFN value between
the two treatments (0.084 for the control, and 0.111 forearly defoliation), but there was a clear positive relationshipbetween the DIFN value and the total amount of radiation
received by the film at that point (Fig. 4b).
Discussion
In this initial evaluation of light sensitive films for quantifying light in the fruiting zone of grapevine canopies, it was
found that the OptoLeaf medium sensitivity R-3D films wereable to characterize differences in the canopy light environment, and provide an absolute measure of cumulative light
exposure changes following the application of a commonlyused defoliation practices. In the first measurement period

Fig. 4: Scale diagram showing position of canopy light measurements and mean transmittance for each ring of the LAI 2200 opticalsensor (a), and relationship between the DIFN parameter at eachfilm position and total radiation received by the corresponding filmin the first field deployment (b).

164

A. Bontempo et al.

following an early defoliation treatment, the films in thecontrol treatment received 9.3 % of the incoming radiation,
which is consistent with the upper range indicated by daily
estimates for bunches located at a similar position within
the fruiting zone of VSP canopies (Louarn et al. 2008). For
the early defoliation treatment the average film exposure
was 12.2 %, and while not significant, deploying the films
3 weeks after the treatment may have reduced exposure
differences due to lateral regrowth. For the second set ofmeasurements immediately following the late defoliation
treatment, the amount of light exposure was increased withthe control treatments receiving 13.4 % compared 20.1 %for the late defoliation treatment. With this second set ofmeasurements the difference in total radiation calculatedusing our own concurrently run linear calibration was alsosignificant. While for the same calculations with the manufacturers calibration was not significant, the total radiation
values were close, and with the significance reflecting aslight difference in the output from the linear and exponential
calibration equations.
Despite the lack of a consistent statistical difference
between the treatments applied, there was a strong linear
relationship between the canopy analyzer DIFN value andcalculated radiation received by the film at each point. For
the R-3D film calibrations there was also a strong linear
relationship with received radiation and good agreement
when the manufacturers calibration was used to calculate
the amount of radiation received by the same films. The
canopy analyzer transmittance values indicate that the lackof large treatment differences, particularly when 75 % of theleaves were removed from the fruiting zone, may insteadreflect the position of the films. While the films were locatedwithin the area of defoliation, the transmittance data show
the defoliation only made a significant difference to theamount of sky visible from lower sensor ring with a fieldof view from 22 to 35° elevation from horizontal. Although
the films were positioned within the fruiting zone in thisstudy, our main interest was to gain experience with using
the system in a viticulture context. If taking the next step tomake a representative measure of the entire fruiting zone,
additional consideration would be needed for film positioning and mounting.
For the other film types tested in the study, the O-1D
film is possibly too sensitive for viticulture applications
unless deployed for very short periods or in heavily shaded
canopies. The rate of fading also appeared more sensitive
to temperature than the other two films. The Y-1W films
showed the least difference between the two calibrations,maintained a near linear response below the 0.6 absorbancevalue recommend by the manufacturer, and may be moresuitable if longer term integration of canopy light conditions
is required. For the R-3D films deployed within the canopy,
fading below the minimum absorbance value occurred inabout 10 days when weather conditions included a numberof overcast days. The rate of fading was faster in periods offull sunshine, suggesting that the R-3D films may be better
suited to shorter deployments of 5-10 days and then removed
as a group before the absorbance of the most exposed films
falls below 0.6 absorbance units. The reason for the slightincrease in film absorbance on some occasions was not

identified. However, as the stated accuracy of portable spectrophotometer is ± 0.05, and up to ± 0.1 absorbance units
under some conditions, this apparent absorbance rise may
reflect a slight difference in the zero point recorded with theblank film at each date.
While we found a better fit with a simple linear regression for the R-3D films, the calibrations provided by the
manufacturer produced radiation values in accordance with
our results, and could be used for converting the change infilm absorbance to cumulative radiation where radiation
data is not available. Regular checks are needed to preventthe most exposed films fading below the recommended
0.6 absorbance units, but in practice this can be difficult to
avoid when many films are deployed in the field. For moregeneral applications, using values down to 0.5 absorbanceunits would not greatly impact on the interpretation basedon our findings, and including some lower points in the calibration allows the linearity below 0.6 to be checked. As theportable spectrophotometer will still provide an absorbancevalue down to 0.5, it also provides some leeway for usingfilms that have faded further than intended. The proximityof a weather station made calibrating a relatively straightforward process in the present study, and if higher accuracy
is required for research purposes, corrects for local weather
conditions and also provides the option to include films withlower absorbance values in the calibration.
As a final point regarding future studies, the nearly equal
contribution of wavelengths above and below 400 nm to film
fading suggests more information about light conditions
between sites could also be obtained with partial screeningof the film with UV-filtering plastic. Films could also beplaced against an opaque surface to screen reflected radiationand provide a directional measure of radiation. Although ourcalibrations were made above a similar ground cover and at
a similar height to the films in the vineyards, pyranometers
are screened from reflected radiation below the horizon someasured values with non-screened films may differ slightly
depending on the amount of radiation reflected from thevineyard floor.
Conclusion
Overall, the light sensitive films used here appear toprovide a reliable approach for estimating and evaluating
cumulative light exposure within the fruiting zone of grapevine canopies. The films can provide an absolute measureof global solar radiation received over the deployment
period, or this value can be expressed as a percentage of
total above canopy radiation if a more standardized value
is required. For research applications the films provide an
option for large numbers of spatial light measurements,
and we suggest the best approach is to deploy a full set offilms and then retrieve them all as the most exposed film
approaches an absorbance threshold defined by the lower
range of the calibrations. The manufacturer recommends avalue of 0.6 for this absorbance threshold, but for the R-3D
films we used for most of this study, values down to 0.5 did
not deviate substantially from the linear relationship withtotal radiation. For industry applications, the films could beEvaluation of photosensitive films for light measurements in the fruiting zone
suited for survey type assessments across vineyards, andprovide a method of linking a fruit composition to a longer
term measure of the canopy light environment. Although
limited to just a few simple comparisons here, we did not
find a large difference between radiation calculated withthe locally produced calibration and the summer calibration
provided by the manufacturer, and suggest the latter could
still be used in the absence of local radiation data.
Acknowledgements
We would like to thank M. Scheidweiler for his support withcoordinating the field site work, and acknowledge manufacturer,
Taisei-Environmental & Landscape Group, Tokyo, Japan, for the
supply of free samples of the three films used for the initial calibration comparisons. The experiment was undertaken as a Vinifera
EuroMaster thesis, and utilized a trial site of the Bordeaux-Adelaide-Geisenheim alliance.

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Received April 13, 2018
Accepted September 4, 2018