PO41
DIFFUSENESS OF ILLUMINATION SUITABLE FOR
REPRODUCING THE SURFACE APPEARANCE OFOBJECTS
Suzuki Mizushima et al.
DOI 10.25039/x47.2020.PO41

Paper accepted for the 5th CIE Symposium on Colour andVisual Appearance

The paper was selected by the International Scientific Committee (ISC) for presentation at the 5th CIE
Symposium on Colour and Visual Appearance, Hong Kong, CN, April 21–22, 2020, which, due to thecorona pandemic, could not take place. The paper has not been peer-reviewed by CIE.

 CIE 2020
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e-mail: ciecb@cie.co.atwww.cie.co.atMizushima, S., Mizokami, Y. DIFFUSENESS OF ILLUMINATION FOR REPRODUCING SURFACE APPEARANCE

PO41

DIFFUSENESS OF THE ILLUMINATION SUITABLE FOR REPRODUCING
THE SURFACE APPEARANCE OF OBJECTS
Mizushima, S.1, Mizokami, Y. 1
Chiba University, Chiba, JAPANsuzuki-mizushima1125@chiba-u.jp
1

Abstract
The appearance of an object is influenced by its material and shape, and the lighting conditions.
Especially, the diffuseness of lighting involves the appearance of material and surface textures.
We investigated the diffuseness of illumination, exhibiting an accurate surface appearancecompared to a memorized material recognition under natural light. We also examined thelighting condition for an ideal material appearance. After pre-observing and memorizing theimpression of objects under daily lighting environments, observers evaluated the appearanceof the objects under different diffuseness lighting conditions. As a result, observers highly rated
moderate diffuseness compared to low and high diffuseness conditions. It implies that a very
low or high diffuseness, which is unfamiliar, is not suitable for reproducing the accurate or ideal
surface appearance. Our results suggest that it is possible to define a diffuseness level for
accurate appearance common to objects with different materials. However, materials should beconsidered for the diffuseness condition for an ideal appearance.
Keywords: Diffuseness of illumination, Material perception, Surface appearance

1 Introduction
The impression of an object changes depending on illumination and many people experience itin everyday life. For example, a product bought under the lighting of the shop may have a
different appearance when it is viewed outside. In recent years, with the evolution of solid-statelamps, including light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs), thedevelopment of lighting with various types of intensity, spectral power distribution, and spatial
distribution has progressed. Besides, lamps with various shapes such as plate-type and curved
designs have also been developed. These lightings could have a great difference in diffusenessfrom conventional lightings. Then they could have a significant impact on the appearance ofobjects. For example, Fig. 1 shows the appearance of an object under illumination with different
diffuseness. Therefore, it is necessary to consider the surface appearance of objects such as
glossiness and roughness when developing and evaluating the lighting environment.
The appearance of an object also largely depends on the material and shape. Studies on texture
appearance have been conducted under various material and surface conditions (Fujisaki et al .,2015; Xiao et al., 2016; Ho et al., 2008). However, regarding the colour of an object, Maloney
et al. have shown that stable colour perception can be achieved without depending on theroughness and glossiness of the surface.

Low diffusenessHigh diffuseness

Figure 1 – Object appearance under lighting with different diffusenessMizushima, S., Mizokami, Y. DIFFUSENESS OF ILLUMINATION FOR REPRODUCING SURFACE APPEARANCEThe effect of surface texture has been extensively studied using images rendered using thebidirectional reflectance distribution function (BRDF) (Fleming et al., 2003; Hartung and
Kersten., 2002; Langer and Bulthoff., 2001; Maloney 2002; Mingolla et al., 2001; Obein et al.,2004). It is a function that expresses the distribution of reflected light concerning the incident
light. Olkkonen & Brainard (2010) showed that the impression of matte objects was stable, but
the impression of glossy objects changed when the lighting environment changed.
Lighting conditions that affect an object’s appearance are related to how the light hits and how
the illumination light spreads. Mizokami et al. (2019) suggested that a rough wavy surface withhigh frequency tends to appear less glossy and smoother as the diffuseness of the light increase.
Moreover, a glossy surface appeared less glossy, and matte one appeared smoother under
diffusive light. Yamazoe et al. (2019) showed that the high diffuseness of illumination increases
the fidelity of the appearance of an object. However, in their experiment, the diffuseness range
was limited to low to middle. Therefore, it was not yet clear how much object’s appearancechanges, and which diffuseness levels reproduce object’s appearance more faithful and ideal.
Here, we investigate the diffuseness of illumination suitable for reproducing the object’s faithfulor ideal surface appearance in a wider diffuseness range.
In Experiment I, we examined lighting conditions which reproduce the faithful and ideal
appearance of the objects in four diffuseness levels. In Experiment II, we added a lower
diffuseness condition and used a new evaluation method to clarify the lighting conditions
suitable for reproducing faithful and ideal appearance.

2 Experiment I
Environment
A viewing booth was constructed by combining a lighting part and an observation part, imitatinga pseudo integrating sphere, as shown in Fig. 2 (a). An observer viewed a stimulus fromobservation holes, as shown in Fig. 2 (b). The experiment was conducted in a dark room. The
lighting part consisted of the combination of an LED lamp (Panasonic EVERLEDS LDA8DA1D;
correlated colour temperature, 6 500 K; colour rendering index, Ra 73) with a duct and a Fresnel
lens. The observation part was made by a white styrofoam sphere with a diameter of 60 cm tocreate a high diffuseness lighting condition.

Figure 2 – (a) Viewing booth, (b) side view of the observation partThe diffuseness of illumination was set at four levels (0.40, 0.55, 0.67, and 0.93) by changing
the length of the duct and the distance between the LED lamp and the lighting port, as shownin Fig. 3. We used the cubic illuminance measurement defined by Cuttle (2014) and the
diffuseness value defined by Xia et al. (2016) as the measurement and evaluation method ofdiffuseness in our experiment. The horizontal illuminance at the position of a stimulus was
unified to 300 lx.

Mizushima, S., Mizokami, Y. DIFFUSENESS OF ILLUMINATION FOR REPRODUCING SURFACE APPEARANCE

Figure 3 – Four-levels of diffuseness conditions

Stimulus
Experimental stimuli are polyresin (diameter 3.4 cm, 83 g, sphere, red), fur charm (diameter 7
cm, 8 g, sphere, grey), wood (4.5 cm, 43 g, cube, brown), and stainless steel (2.6 cm, 35 g,cube, silver). Figure 4 shows the experimental stimulus under each illumination.

Polyresin

Fur

Wood

Stainless

Diffuseness

0.40

0.56

0.67

0.93

Figure 4 – Experimental stimulus of Experiment I. Polyresin, fur, wood, and stainless steel inorder from the top. Diffuseness level 0.40 (Low), 0.56 (Middle), 0.67 (High), and 0.93 (Very High)in order from the left.

Evaluation methods
The semantic differential scale (SD) method (3-0-3) with ten items was used to evaluate theimpression of the object: “Weight”, “Hardness”, “Naturalness”, “Roughness”, “Value”,“Glossiness”, “Preference”, “Visibility”, “Brightness”, and “Transparency”. In addition to theimpression evaluation of the SD method, observers selected the best illumination condition tofaithfully produce the object’s appearance in their memory (fidelity illumination) and the best
illumination condition to ideally produce the feature of the object’s surface (ideal illumination).

Procedure
As a pre-observation condition, observers memorized the appearance of the stimuli by touching
and viewing them at a window and in an office room for 5 minutes each for a total of 10 minutes
in two separate days. The illuminance at the window ranged from 432 lx to 5340 lx. The room
lighting was white fluorescent lamps (National FHF32EX-NH; CCT 5000K; Ra 84), and theMizushima, S., Mizokami, Y. DIFFUSENESS OF ILLUMINATION FOR REPRODUCING SURFACE APPEARANCE

illuminance ranged from 112 lx to 497 lx. As a pre-evaluation, the observers evaluated theimpression of the stimuli after viewing them in the office room.
In the main evaluation, the observers viewed the stimulus in four different lighting conditions in
the viewing booth, and the impression evaluation was performed. The observation time of thestimulus was unlimited, and the lighting conditions’ order was randomized. After viewing thestimuli, the observer selected the fidelity illumination and the ideal illumination . The preevaluation and the main evaluation were combined into one session, and five sessions wereconducted.

2.5 Observer
Three male and two female observers in their twenties participated.

2.6 Result and Discussion
The profile of the impression evaluation of observer A is shown in Fig. 5 (a). The averages ofthe selection rate of the fidelity illumination and ideal illumination o f all observers are shown in
Fig. 5 (b) and (c). Condition of very high diffuseness tends to change the impression of polyresinand stainless objects. In the faithful selection, polyresin has a high rating for low diffuseness,whereas other stimuli have a high rating for middle diffuseness. The ideal choice showed no
consistent tendency overall.
For fidelity illumination, the selection ratio of “Middle (0.55)” was the highest. This diffusenesscondition is the closest to the average diffuseness (0.51) of the pre-observation condition. For
ideal illumination, polyresin and stainless stimuli had a high selection rate for low diffuseness.
Fur and wood had a high selection rate for high diffuseness.
The evaluation results of the SD method at the pre-evaluation show the characteristics of thestimuli memorized in the daily environment. Polyresin and stainless stimuli have a high scorein the “glossiness” item.
The range of diffuseness during observation did not cover low diffuseness. We needed a more
extensive diffuseness range of illumination to investigate a comprehensive relationship between
the appearance of objects and the illumination. Therefore, we conducted Experiment II.

Mizushima, S., Mizokami, Y. DIFFUSENESS OF ILLUMINATION FOR REPRODUCING SURFACE APPEARANCE

Polyresin

Fur

Wood

Stainless

Opacity
Dark
Invisible
Dislike
Matte
CheesySmoothUnnatural
Soft
Light
-3

-2

-1

0

1

2

3

-3

-2

-1

0

1

2

3

-3

-2

-1

0

1

2

3

-3

-2

Selection rate

1

2

3

＊＊ P < .05

(a)
1

1

1

1

0,8

0,8

0,8

0,8

0,6

0,6

0,6

0,6

0,4

0,4

0,4

0,4

0,2

0,2

0,2

0,2

0

0

0
0,5

0

━ Original ━ Low ━ Middle ━ High ━ Very High

Score

0

-1

Transparent
BrightVisibleLike
GlossyLuxuryRoughNatural
hard
heavy

1

**

0

0,5

0
0

1

0,5

1

0

0,5

1

0

0,5

1

● Low ■ Middle ▲ High ◆ Very High

Selection rate

(b)
1

1

1

1

0,8

0,8

0,8

0,8

0,6

0,6

0,6

0,6

0,4

0,4

0,4

0,4

0,2

0,2

0,2

0,2

0

0

0

0

0,5
Diffuseness

1

0

0,5

1

0
0

0,5

1

● Low ■ Middle ▲ High ◆ Very High(c)
Figure 5 – Results of Experiment I. (a) The profile of the impression evaluation result ofobserver A. (b) the average selection rate of all observers for the fidelity illumination. (c) the
average selection rate of all observers for the ideal illumination.

3 Experiment II
Environment
The viewing booth was the same as in Experiment I, except that we added a low diffuseness condition
by covering the inside of the upper half of the styrofoam sphere with a black cloth to extend the
diffuseness range. Therefore, we tested five diffuseness conditions (0.26, 0.40, 0.55, 0.67, and 0.93) inExperiment II as shown in Fig. 6.

Mizushima, S., Mizokami, Y. DIFFUSENESS OF ILLUMINATION FOR REPRODUCING SURFACE APPEARANCE

Figure 6 – Five-level diffuseness conditions

Stimulus
The same objects as in Experiment I was used. Figure 7 shows the state of the stimulus undereach illumination.

Polyresin

Fur

Wood

Stainless
Diffuseness

0.23

0.40

0.56

0.67

0.93

Figure 7 – Experimental stimulus of Experiment II. From top to bottom: Polyresin, fur, wood,and stainless steel. From left to right: Diffuseness level 0.23 (Very Low), 0.40 (Low), 0.56
(Middle), 0.67 (High), and 0.93 (Very High).

Evaluation methods
Under each diffuseness condition, observers evaluated using ten steps from 1 to 10 for the twoitems regarding the fidelity to the stimulus appearance memorized in the daily environment andhow much the stimulus appearance was ideally represented (appearance evaluation).

Procedure
The pre-observation was performed in the same process as Experiment I. The illuminance at
the window ranged from 51 lx to 4 650 lx. The illuminance inside the room ranged from 150 lx
to 454 lx. After viewing the stimulus for 10 minutes in the same environment as in the preobservation, the observer viewed the stimulus in the viewing booth under five lighting conditions.
Then, the observer performed appearance evaluation. The observation time was unlimited, andthe order of the illumination conditions was randomized. The order of presenting stimuli wasalso randomized, and all four types of stimuli were evaluated in one session. Five sessions
were conducted.

Mizushima, S., Mizokami, Y. DIFFUSENESS OF ILLUMINATION FOR REPRODUCING SURFACE APPEARANCE

ObserverObservers were two males who participated in Experiment I and two females who did notparticipate in Experiment I.

Result and Discussion
Figure 8 (a) and (b) show the average score of the fidelity and ideal lighting conditions for allobservers and all stimuli. Low lighting conditions obtained a high score in the ideal evaluation
for stainless, and medium score for other evaluations. Besides, Middle and High diffusenesslighting conditions had a high score in the fidelity evaluation and the ideal evaluation.

Score

Polyresin

Fur

Stainless

Wood

10

10

10

10

7

7

7

7

4

4

4

4

1
0

0,5

1

1

1
0

0,5

1
0

1

0,5

1

0

0,5

1

0

0,5

1

━ Very low ● Low ■ Middle ▲ High ◆ Very High

Diffusivity

Score

(a)
10

10

10

10

7

7

7

7

4

4

4

4

1

1
0

0,5
Diffusivity

1

1

1
0

0,5

1

0

0,5

1

━ Very low ● Low ■ Middle ▲ High ◆ Very High(b)
Figure 8 – Results of Experiment II. (a) the average score of the fidelity illumination for allobservers. (b) the average score of the ideal illumination for all observers.

4 Discussion
The results of appearance evaluations are different in Experiment I and II, probably because
the evaluation method was different. In Experiment I, selecting the most faithful and ideal
lighting one by one caused a large variation in evaluation. On the other hand, it was able toobtain quantitative evaluations for all stimuli in Experiment II. It was found that the scores of
middle diffusenesses that were not selected in Experiment I were also high in all stimuli. Thereis variability in evaluation at very low and high diffuseness. It is considered that the results ofExperiment II, including a large amount of data, are highly reliable.
In the experiment by Yamazoe et al. (2019), the diffuseness range was from 0.18 to 0.43, andthe fidelity increased as the diffuseness increased. In the present experiments, Very Low (0.23)
and Low (0.40) were within that range, and similar results to Yamazoe et al. were obtained forall objects except for fur.
The differences in evaluation between stimuli in Experiment II are similar for polyresin andstainless (P-S), and for fur and wood (F-W) in general, as shown in the average data in Table
1 and 2. In the impression evaluation in Experiment I, polyresin and stainless (P-S), and fur andwood (F-W) also had similar evaluations in average data in terms of glossiness and roughness
item, as shown in Tables 3 and 4. These results suggest that objects with similar property (e.g.

Mizushima, S., Mizokami, Y. DIFFUSENESS OF ILLUMINATION FOR REPRODUCING SURFACE APPEARANCE

both glossy polyresin and stainless) give shimilar impression. It can be considered that thesurface appearance was influenced by the surface properties of the object .
Table 1 – Difference in appearance evaluation between stimuli (Fidelity) in Experiment II
P: Polyresin, F: Fur, W: Wood, S: Stainless

Very Low

Low

MiddleHigh

Very High

Average

P-F

0.35

1.00

0.55

1.05

2.90

1.17

P-W

0.35

0.25

0.60

0.80

2.65

0.93

P-S

0.70

0.10

0.05

0.85

0.65

0.47

F-W

0.00

0.75

0.05

0.25

0.25

0.26

F-S

1.05

0.90

0.60

1.90

3.55

1.60

W-S

1.05

0.15

0.65

1.65

3.30

1.36

Table 2 – Difference in appearance evaluation between stimuli (Ideal) in Experiment II

Very Low

Low

MiddleHigh

Very High

Average

P-F

0.20

1.35

0.05

0.15

2.75

0.90

P-W

0.30

0.70

0.15

0.15

1.90

0.64

P-S

0.75

0.20

0.30

0.70

1.05

0.60

F-W

0.10

0.65

0.10

0.00

0.85

0.34

F-S

0.95

1.55

0.35

0.85

3.80

1.50

W-S

1.05

0.90

0.45

0.85

2.95

1.24

Table 3 – Difference in impression evaluation between stimuli (Glossiness) in Experiment I

Original

Low

MiddleHigh

Very High

Average

P-F

2.84

2.68

2.32

2.30

1.90

2.41

P-W

2.40

2.66

2.74

2.28

1.44

2.30

P-S

0.80

0.28

0.22

0.10

0.46

0.37

F-W

0.44

0.02

0.42

0.02

0.46

0.27

F-S

3.64

2.96

2.54

2.40

1.44

2.60

W-S

3.20

2.94

2.96

2.38

0.98

2.49

Table 4 – Difference in impression evaluation between stimuli (Roughness) in Experiment I

Original

Low

MiddleHigh

Very High

Average

P-F

2.60

2.40

2.40

2.42

2.36

2.44

P-W

1.54

1.86

1.42

1.66

0.90

1.48

P-S

0.44

0.14

0.08

0.00

0.06

0.14

F-W

1.06

0.54

0.98

0.76

1.46

0.96

F-S

3.04

2.26

2.48

2.42

2.30

2.50

W-S

1.98

1.72

1.50

1.66

0.84

1.54

Mizushima, S., Mizokami, Y. DIFFUSENESS OF ILLUMINATION FOR REPRODUCING SURFACE APPEARANCE

The reason why the middle diffuseness was highly evaluated would be because it was within
the diffuseness range at pre-observation, where observers memorized the appearance ofobjects. The other possibility would be related to the naturalness of lighting conditions. Fleming
et al. (2003) revealed that the reflection properties of objects in familiar environments are easier
to distinguish than in unfamiliar environments. It can be considered that the moderate
diffuseness was preferred because it is commonly seen in our ordinary l ife. Our results suggestthat unfamiliar lighting conditions (very low and very high diffuseness) did not correctly
represent the appearance of objects.

5 Conclusion
In this study, the observers viewed objects with different materials, shapes, and textures. Theyevaluated the impression of those objects, as well as the fi delity and the ideality of objects’appearance under different diffuseness levels of illumination. The results showed that
illumination with middle diffuseness reproduced the appearance of objects faithful and ideally,and extremely low and high diffuseness were not. It was also suggested that the diffuseness,
which ideally reproduces the surface appearance, would be different in object’s properties.
To summarize, we showed that the impression of surface appearance was influenced by thesurface properties of the object and lighting distribution. Our results suggest that it is possible
to define a diffuseness level for accurate appearance common to objects with different materials.
However, the difference in the material should be considered for the diffuseness condition for
an ideal appearance to realize the ideal appearance.

Acknowledgments
Supported by JSPS KAKENHI JP16K00368 and JP19H04196.

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