Exploring Asymmetric Lens–Total Internal Reflection (AL–TIR) Optics for Uniform Ceiling Illumination in Interior Lighting

Comments 1: Please explain the general idea of AL-TIR in the abstract by adding one more sentence about it. It is described in the introduction but should also be included in the abstract. Moreover, I think that symbols in abstracts like "U_tled-direct" are not a good idea and should be removed.

Response 1: Thank you for this suggestion. We have added a new sentence to the abstract explaining AL–TIR technology and replaced symbols with descriptive text:

-        “AL–TIR technology employs asymmetric lenses with total internal reflection to efficiently redirect light, achieving a more even and controlled ceiling illumination suitable for interior environments." (Page 1, Line 14 - 16).

-        “direct illuminance uniformities” – Page 1, line 19

Comments 2: After reading the entire article, it is not clear why the idea of ​​HCL was briefly described in the introduction. The presented research and solutions are rather linked to proper interior lighting design, visual impressions, or lighting esthetics rather than the pure HCL conception. Please clarify or slightly correct the introduction.

Response 2: We clarified in the introduction that spatial comfort and visual impressions are essential elements of HCL, and while our design is not a full HCL system, it aligns with HCL principles through its contribution to visual comfort.

-        " In addition to these biological aspects, spatial comfort and visual impressions—achieved by mimicking sky-like light distribution—are critical for enhancing occupant well-being and aligning with the aesthetic goals of HCL principles, as noted by Münch (2020) [5] and De Vries (2021) [6]. Chraibi et al. (2017) [7] demonstrated that uniform wall luminance enhances visual comfort, highlighting the importance of consistent lighting distribution in interior environments​. The study by Klir et al. (2023) [8] highlighted that sky-like interior lighting settings are preferred by users, as they enhance visual comfort and well-being. Yang et al. (2022) [9] reported on a 3D uniform light field effectively supports health by facilitating SAD therapy and circadian rhythm adjustment, enhancing user convenience and comfort in a healthy residential environment." (Page 2, Lines 34-43)

Comments 3: The flowchart presented in Figure 1 needs to be clarified. How do you select paths described as "bad"? Please explain it.

Response 3: The updated flowchart uses "Adjust" instead of "Bad," suggesting points in the process where adjustments are needed to improve alignment with design goals. Page 3, line 88

Comments 4: There is a typo in L.85. The unit of illuminance is lm/m2 = lx. Please amend it.

Response 4: We amended it! Page 3, line 96

Comments 5: Formulas (1) and (2). The typical symbol for illuminance is E. The symbol I is usually the luminous intensity, which may mislead the reader. Please amend it.

Response 5: We replaced symbol I with E in all formula

Comments 6: L.141 and L294-295 contain the terminology issue. The words "luminaire" or "lighting fixture" should be used instead. Please amend it.

Response 6: We replaced all terms “lamps” with “luminaires”

Comments 7: Figure 3 – why the black line is not finished? Please amend it.

Response 7: Thank you for your insightful observation regarding the unfinished black line in Figure 3. The reason the black line does not extend to 90 degrees is twofold. First, as per Equation 3, the numerical calculation of tan or arctan becomes imprecise as it approaches 90 degrees, which can introduce errors. Second, the ceiling width (W) is limited to approximately 4 meters, as shown in Figure 2(b). For the black curve corresponding to k=1 the calculated angle extends beyond this region of interest, making further calculation or correction unnecessary.

We appreciate your attention to detail and hope this explanation clarifies our approach.

Comments 8: There needs to be a text referring to Figure 4 in the entire paper. Please amend it.

Response 8: Thank you for pointing this out. The reference to Figure 4 is indeed located in lines 176-177 (Page 6, line 186 in the revised manuscript), though it may not have been clearly visible.

Comments 9: L.224 Please specify the general lighting requirements for the ceiling illumination.

Response 9: Thank you for your suggestion. We have added a sentence on lines 243-249 to clarify your concerns based on the report of De Boer.

-        “The two lighting researchers De Boer and Fischer stated in 1978 that at a horizontal illuminance of 1000 lx, a luminance for the ceiling of 200 cd/m² and for the walls of 100 cd/m² are preferred [24]. At a horizontal illuminance of 500 lx (as defined as a minimum by EN 12464-1:2021 [25]), the above values can be estimated as about 210 cd/m² (ceiling) and 70 cd/m² (walls). For a possible control of the next generation of intelligent luminaires, this means that ceiling luminance should remain almost constant at 200 - 210 cd/m²” (Page 8, lines 243-249).

Comments 10: What software did you use to create the color LIDs in Figure 5?

Response 10: Thank you for your question. The color LIDs (Luminous Intensity Distributions) in Figure 5 were created using TracePro (Lambda Research Co.), which allowed us to visualize the distribution of light intensity across different angles in a color-coded format.

Comments 11: Figure 6 – Why there is no 1 or 100% if the LIDs are normalized? Please explain.

Response 11: Thank you for your observation. We have updated Figure 6 with new LIDs normalized to 100% for clarity.

-        Figure 6 (Page 8, line 257) and figure 7 (Page 9, line 285)

Comments 12: L.291-293 Please clarify that the only direct illumination component was analyzed.

Response 12: Thank you for your observation. We have revised the text on Lines 303-307 to clarify that only the direct illumination component was analyzed. The reflectance values for ceiling, wall, and floor materials were included solely to visualize the ceiling’s appearance in the simulation and do not affect the direct illuminance calculations.

-        “Typical materials for the ceiling, wall, and floor were chosen with reflectance values of 85%, 70%, and 60%, respectively, to accurately visualize the ceiling's appearance in the simulation. However, these reflectance values were not factored into the direct illuminance calculations (E), ensuring that only the direct illumination component was analyzed”. (Page 10, Lines 308-312).

Comments 13: L.295 Luminaires were suspended 0,3 m from the ceiling. Please change it.

Response 13: The new sentence was replaced specifies that the luminaires are fixed to the wall at a set distance from the ceiling.

-        “Two sets of six luminaires (a total of 12) were mounted on opposite sides of the room, positioned 0.3 meters below the ceiling (H = 0.3 m)”. (Page 10, Lines 313-314)

Comments 14: Section 4.3. Please specify that illuminance uniformity is the relation between minimum and average illuminance values.

Response 14: Thank you for your comment. The definition of illuminance uniformity as the ratio between minimum and average illuminance values is provided in Section 4.2. We appreciate your attention to detail.

-        “The illuminance uniformity U_o was employed as a measure of lighting quality, defined by the ratio of minimum to average illuminance “. (Page 11, lines 328-330.)

Comments 15: L.378-387 is a very nice part presenting the association between simulations and measurements. It also proves the correctness of the designed lighting solution. Good job!

Response 15: Thank you for your kind feedback. We’re glad that you found the association between simulations and measurements to be clear and that it supports the validity of our lighting design. We appreciate your encouraging words!

Comments 16: The literature review can be wider including the most important items related to the interior lighting. However, it seems to be enough in the area of applied science.

Response 16: Thank you for your suggestion. We appreciate your feedback and are glad that the literature review meets the standard for applied science. We have added 6 recent works related to our study. References 7-10, 24-25.

Comments 1: In figure 2(a), there are two ceilings. Which one is correct?

Response 1: Thank you for your attention to detail. We have replaced it with the new corrected Figure 2. (Page 2, line 98)

Comments 2: In a typical room, there are windows, doors, and different furniture. They are necessary for a room. However, how to arrange these parts in the authors’ 2D model?

Response 2: Thank you for your question. For simplicity and generality, we did not include specific elements like windows, doors, or furniture in our 2D model for direct ceiling illumination analysis, as these do not significantly affect direct illumination. This approach allowed us to focus on core lighting performance.

However, in the case study in Section 4.3, we simulated a realistic room by considering surface properties and used luxmeters and luminance meters to evaluate reflectance, accounting for variations and errors. This ensured our design's effectiveness in practical applications.

Comments 3: In my view, closely mimicking natural light is not a necessary to improve human health and productivity. It seems to cost too much to fulfill the installation of the AL-TIR optics in a room, which actually people just need for sleep.

Response 3: Thank you for your valuable feedback. While closely mimicking natural light is not strictly necessary, our AL-TIR optics provide a powerful tool for architects who wish to create such environments. We believe that since humans evolved in natural lighting conditions, replicating this can contribute positively to well-being, aligning with Human-Centric Lighting (HCL) principles. Regarding cost, it varies based on specific installation factors, but we have successfully implemented this system in practical settings, where cost efficiency remains a priority for customers. This suggests that, despite initial concerns, there is demand and feasibility for such solutions.

Comments 4: Will the ceiling reflect light completely?

Response 4: Thank you for your question. No, the ceiling will not reflect light completely, as the best reflective interior materials generally have reflectance values below 90%. However, the TOUF (Total Optical Utilization Factor) of our design is effectively 100%, which might seem high. This is because, in our setup, multiple reflections within a white environment can enhance the overall illumination, similar to the behavior observed in an integrating sphere, where the Amplification Factor (AF) can be greater than 1. This effect allows our design to achieve high optical efficiency despite the limitations of material reflectance.

Comments 5: 5.4.4 discussion summary seems not to be necessary as there are conclusion section immediately following.

Response 5: We eliminated the "Discussion Summary" section to streamline the content and avoid duplication with the conclusions.

We integrated essential elements from the "Discussion Summary" into the "Conclusions," such as:

-        “which achieved nearly ideal performance and cost-effectiveness”. (Page 13, line 417)

-        “This design also reduces shadows from ceiling fixtures and floor-standing objects, enhancing spatial comfort by using a uniformly illuminated large-area ceiling as an indirect light source”. (Page 13, lines 420-422)

-        “The integration of even ceiling lighting with adjustable LED spectra, color correlated temperature, and high color rendering index creates a lighting environment that closely mimics natural conditions, supporting HCL solutions”. (Page 14, lines 426-429)

Comments 1: The manuscript deals with the design of a luminary designed to improve the non-directionality of a ceiling. This work is based on an asymmetric lens.

Comments 2: The authors compare several lenses available already in the literature with the proposed design and claim 100% of the total utility factor for their design compared to 88% at most for the literature.

Response 1, 2: Thank you for your accurate summary of our manuscript. We appreciate your understanding of the focus on asymmetric lens design and the comparison with existing solutions.

Comments 3: In Figure 7, the authors present the luminous intensity only up to 85 degrees for K3. This does not seem fair compared to the other two designs presented in such a graph.

Response 3: Thank you for your observation regarding Figure 7. We limited the luminous intensity calculation to 86 degrees for two main reasons. First, the accuracy of numerical calculations involving tan and arctan functions declines near 90 degrees, which can introduce errors. Second, as shown in Figures 2b and 3 (left), the luminous intensity for K=3 is nearly zero beyond 86 degrees. Therefore, extending the graph further would not add useful information, and the comparison remains valid with minimal error.

Comments 4: As the authors already state in Figure 7, the asymmetric design presents a reduced luminous intensity (around 7%), which practical implications have the reduced luminous intensity shown by the asymmetric design compared to the symmetric.

Response 4: Thank you for your question regarding the practical implications of the reduced luminous intensity in the asymmetric design compared to the symmetric one. While the peak luminous intensities of the asymmetric (L3, AL-STIR) and symmetric (L4, AL-ATIR) designs are not significantly different, Figure 7 shows that in the range from 0 to about 64 degrees, the intensity of the L3 luminaire is notably lower than that of L4. This discrepancy results in a reduction of direct uniformity from 0.85 (AL-ATIR) to 0.56 (AL-STIR), leading to visible black and white bands, as illustrated in Figure 9c. These variations in uniformity can affect the overall lighting quality and visual comfort.

Comments 5: Figure 8, parts b, d, and f have markers that are difficult to see, and the color scale is also hard to distinguish. I suggest adding a bigger color scale or adding only the relevant portion of the scale (from navy blue to yellow and omitting the black-purple and orange-white regions), the similar case for Figure 9, parts b, d, and f.

Response 5: Thank you for your valuable feedback on Figures 8 and 9. We have revised the figures as per your suggestions. Specifically, we have enlarged the color scale and adjusted it to include only the relevant portions (from navy blue to yellow) to improve visibility, omitting the less significant ranges (black-purple and orange- white). Additionally, the markers in parts b, d, and f of both figures have been enhanced for better clarity. We hope these adjustments address your concerns and improve the overall readability of the figures.

Comments 6: Please provide a more comprehensive explanation of the selection of LED lamp configurations to compare with the presented design.

Response 6: Thank you for your comment regarding the selection of LED lamp configurations for comparison. We added an explanation.

-        “In summary, we selected the Tube LED as a reference configuration due to its popularity and widespread use as a standard luminaire. The NFLED was chosen for its reputation as one of the top-performing wall-washing luminaires. The AL0 and AL6 LED luminaires were included based on their use in previous projects, allowing for direct and straightforward comparison. Finally, the AL-STIR LED was included based on recent advancements proposed by He XY [15] in asymmetric lighting design”. (Page 12, Lines 379-384)

Comments 7: The sub-image labels are of different sizes for the same figure; for example, figure 8 labels (a) and (b) look weird.

Response 7: Thank you for pointing out the inconsistency in the sub-image labels for Figure 8. We have adjusted the labels to ensure that they are uniform in size and style across the entire figure. This change should improve the visual consistency and readability of the sub-images. We appreciate your attention to detail

Comments 8: In Figure 9, parts c and d seem to have repeated information. I suggest joining parts a and b and using the same format for parts c and d.

Response 8: Thank you for your observation regarding Figure 9. We understand your concern about the potential repetition between parts (c) and (d). However, we would like to clarify that the appearance images on the left (including part (c)) are essential because they provide a visual impression of lighting uniformity on the ceilings and walls, which is influenced by surface reflectance properties. In contrast, the illuminance color maps on the right (such as part (d)) show the distribution of direct illuminance, which is not affected by surface material properties. To address your feedback, we have removed the contour maps from Figures 8 and 9 to minimize repeated information, as recommended.

Comments 9: The equation size does not seem to fit the journal style.

Response 9: Thank you for your comment. We have adjusted the equation to align with the journal's formatting requirements.

Comments 10: The reference section is based on the presented work.

Response 10: Thank you for your feedback regarding the reference section. We have expanded the literature review by adding 5 recent works to significant studies in interior lighting, which we believe will provide readers with a more complete understanding of the field. (Refs. 7-10, 24-25).